Monodisperse nanoparticles for catalysis and nanomedicine

Michelle Muzzio a, Junrui Li a, Zhouyang Yin a, Ian Michael Delahunty b, Jin Xie b and Shouheng Sun *a
aDepartment of Chemistry, Brown University, Providence, Rhode Island 02912, USA. E-mail: ssun@brown.edu
bDepartment of Chemistry, University of Georgia, Athens, Georgia 30602, USA

Received 18th July 2019 , Accepted 3rd August 2019

First published on 6th August 2019


The growth and breadth of nanoparticle (NP) research now encompasses many scientific and technologic fields, which has driven the want to control NP dimensions, structures and properties. Recent advances in NP synthesis, especially in solution phase synthesis, and characterization have made it possible to tune NP sizes and shapes to optimize NP properties for various applications. In this review, we summarize the general concepts of using solution phase chemistry to control NP nucleation and growth for the formation of monodisperse NPs with polyhedral, cubic, octahedral, rod, or wire shapes and complex multicomponent heterostructures. Using some representative examples, we demonstrate how to use these monodisperse NPs to tune and optimize NP catalysis of some important energy conversion reactions, such as the oxygen reduction reaction, electrochemical carbon dioxide reduction, and cascade dehydrogenation/hydrogenation for the formation of functional organic compounds under greener chemical reaction conditions. Monodisperse NPs with controlled surface chemistry, morphologies and magnetic properties also show great potential for use in biomedicine. We highlight how monodisperse iron oxide NPs are made biocompatible and target-specific for biomedical imaging, sensing and therapeutic applications. We intend to provide readers some concrete evidence that monodisperse NPs have been established to serve as successful model systems for understanding structure–property relationships at the nanoscale and further to show great potential for advanced nanotechnological applications.


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Michelle Muzzio

Michelle Muzzio is a Ph.D candidate in the Department of Chemistry at Brown University where she works with Prof. Shouheng Sun. She has previously received her B.S. in chemistry from Iona College where she worked on model biological membranes with Prof. Sunghee Lee, as well as B.A. in English Literature. Her research interests in the Sun Lab have been the synthesis of Pd-based alloy nanoparticles for greener chemical syntheses of value-added molecules and materials.

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Junrui Li

Junrui Li received his Ph.D from the Department of Chemistry at Brown University under the advice of Prof. Shouheng Sun, and B.S. and M.E. in applied chemistry from the Wuhan University of Technology. His research interests include the synthesis of mesoporous materials and nanomaterials for catalytic applications, such as fuel cell reactions, CO2 reduction, and biomass conversion. He also works on proton conductive membranes for fuel cell applications.

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Zhouyang Yin

Zhouyang Yin received his B.S. degree in chemistry from Nanjing University in 2015. Since then, he has been a Ph.D. candidate in Brown University under the supervision of Prof. Shouheng Sun. He is conducting research on the electrochemical carbon dioxide reduction reaction.

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Ian Michael Delahunty

Ian Delahunty graduated from Denison University in the Spring of 2016 with a B.S. in Biochemistry. In the Fall of 2016 Ian joined the Department of Chemistry at the University of Georgia in the lab of Prof. Jin Xie to pursue his Ph.D. and is currently a candidate for the degree. His research interests include vitamin-based drug delivery methods and their effects on cellular vitamin metabolism and cancer therapeutic potential.

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Jin Xie

Prof. Jin Xie obtained his Ph.D. in Chemistry from Brown University in 2008. He worked as a postdoc at the Molecular Imaging Program at Stanford between 2008 and 2009. He then moved to the National Institute of Biomedical Imaging and Bioengineering (NIBIB) as a research fellow. In 2011, he joined the faculty of the Department of Chemistry, University of Georgia, as an assistant professor. He was promoted to associate professor with tenure in 2017. His research interests include nanoparticle-based drug delivery, radiotherapy, and photodynamic therapy.

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Shouheng Sun

Prof. Shouheng Sun received his Ph.D. degree in chemistry from Brown University in 1996. He had been a postdoctoral fellow from 1996 to 1998 and a research staff member from 1998 to 2004 at the IBM T. J. Watson Research Center. He joined the Department of Chemistry at Brown University in 2005. He is now a Professor of Chemistry and Engineering at Brown. His research interests are in nanomaterials synthesis, self-assembly, and applications in catalysis and nanomedicine.


Introduction

Nanoparticles (NPs), commonly referred to as particles with dimensions less than 100 nm, are prevalent in nearly every facet of our lives. From potential applications in sustainable energy and therapeutics, to practical uses in art, agriculture, and chemical industries, the widespread use of “nano-” has been achieved but research challenges still persist. These drive the need to control the synthesis of NPs in order to realize the desired functionalities that are essential for applications.1 Interestingly, the applications of NPs have been around for hundreds of years, without a detailed understanding of what the nanoscale even was. For example, nanostructured Cu, Ag and Au were used to colour ceramics and glass 500–1000 years ago.2 Only within the last twenty years, however, has the technology been available for careful design of NPs, and with this, the desire to control the NPs with precise dimensions and enhanced properties has taken flight.

In the endeavour to prepare better NPs, monodispersity has risen as a measure of quality control. For NPs to be defined as monodisperse, the standard deviation in their diameter (or in one dimension) should be less than ten percent.3 Monodisperse NPs are ideal model systems for understanding property tuning and optimization at the nanoscale. While it is well-known that nanomaterials behave very differently from bulk materials,4 fine tuning of NP properties can only be possible when the NPs are monodisperse and structure–property relationships can be understood.

As the size of NPs decreases, the percentage of surface atoms increases exponentially, as summarized in Fig. 1A. Chemically, this increase in surface atoms provides more binding sites in the same molar amount of NPs, an important factor of any NP application, especially catalysis in which the surface of NPs allows for chemical reactions, complementing catalyst–support interactions.5 Physically, the size of NPs is also critical to determine their optoelectronic and magnetic properties.1b,6 The size effects of monodisperse NPs on their properties are evident in semiconducting NPs, also referred to as quantum dots (QDs), in their band gap/optics tuning (Fig. 1B), where larger QDs exhibit a narrower band gap, red-shifting the wavelength of absorption/emission light.7 In magnetic NPs, size is also paramount in determining magnetic coercivity and magnetization.8 NPs at a material-specific critical diameter (Dc) allow supporting only a single magnetic domain within which magnetization reversal is decided by magnetocrystalline anisotropy energy, leading to the increase in magnetic coercivity, as illustrated in Fig. 1C. The magnetization direction of NPs smaller than Dc is subject to thermal agitation, and at a material-specific dimension Ds, they become superparamagnetic and show no coercivity. The increased number of surface atoms in smaller NPs further degrades the magnetization values (Fig. 1D) due to the presence of a larger fraction of surface atoms which are often magnetically “silent” due to the surface oxidation/binding state. Therefore, a rigorous demand for monodisperse NP syntheses has arisen to meet the rising standards of NP functionality in every one of their widespread applications.


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Fig. 1 (A) The relationship between the NP size and the percentage of atoms on the surface or within the bulk of the NP, (B) band gap tuning through NP size control in QDs (adapted from ref. 7b with permission, Copyright 2010 John Wiley & Sons, Inc.), (C) coercivity dependence on NP size in magnetic NPs (adapted from ref. 8a with permission, Copyright 1996 American Chemical Society), and (D) the effect of NP size on magnetization, using iron oxide NPs as a model (adapted from ref. 8b with permission, Copyright 2011 American Chemical Society).

Recent advances in NP synthesis have reached a level where most NPs can now be made monodisperse, allowing a deeper fundamental understanding of NP structure–property relationships for various applications, including those related to our daily life in energy, medicine and the environment. This review focuses on the general syntheses of monodisperse NPs and the application of these monodisperse NPs in catalysis and biomedicine, as outlined in Fig. 2.


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Fig. 2 Outline of the content of this review on monodisperse NPs and their applications in catalysis and nanomedicine.

This review first summarizes the general concept of synthesizing monodisperse NPs under both thermodynamic and kinetic growth conditions to achieve the desired NP size and shape control. It then highlights the applications of monodisperse NPs as catalysts for fuel cell reactions, the electrochemical reduction of carbon dioxide (CO2) and greener chemical syntheses of value-added chemicals. It further highlights the applications of monodisperse NPs as probes for biomedical imaging, sensing and therapy. A great number of excellent publications on monodisperse NPs have been available, which makes any comprehensive review writing very challenging. In this review, we chose to use some examples published from our own group, plus a few representative examples from other publications to highlight the key ideas of the syntheses of monodisperse NPs and their applications in catalysis and biomedicine.

Synthesis of monodisperse NPs

Monodisperse NPs are normally synthesized via solution phase reactions in which NP nucleation and growth can be better controlled, as illustrated in Fig. 3.9 There are two main types of syntheses which can yield monodisperse NPs: burst nucleation and seed-mediated growth. In each synthetic process the general components typically needed are a solvent, monomers (or NP precursors), a surfactant (also called a capping agent/stabilizing agent/ligand) and a reductant if the reduction of NP precursors is necessary.10 The solvent is chosen as the reaction medium in which the NP precursor can react uniformly for NP formation, and the surfactant is present to react with the NP surface to form a layer of coating for NP stabilization during and after NP synthesis. There are many parameters which may be tuned to obtain stable monodisperse NPs. Therefore, care must be taken in the synthesis to ensure the reaction conditions are well-controlled to prepare NPs with certain sizes and shapes. To achieve burst nucleation, the growing monomers should be generated in a very short period of time to allow their concentration to reach the nucleation threshold, over which nuclei are formed.11 Nucleation of NPs is not thermodynamically favoured and external energy is required to accomplish this process. Once nuclei are formed, monomers can be added to these nuclei, and the reaction enters the growth stage. This is a thermodynamically favoured process as the occurrence of the extra surface binding lowers the system free energy. This is also an important reason why a surfactant must be present in the reaction solution to slow down or even stop this spontaneous growth process to ensure the NPs at a certain size range can be stabilized in the reaction solution. In this process, many factors can affect the NP growth, including the NP surface energy, growth rate, and surfactant chemistry.9,12 Under thermodynamic growth conditions, stable polyhedral NPs are often obtained. For example, monodisperse Au NPs are synthesized in tetralin via reduction of HAuCl4 in the presence of oleylamine (Fig. 4A).13 Magnetic Co NPs are synthesized via thermal decomposition of Co(CO)8 in the presence of dioctylamine and oleic acid (Fig. 4B).14 This metal carbonyl decomposition chemistry has also been used to prepare Fe NPs from the decomposition of Fe(CO)5 in an oleylamine solution of 1-octadecene (Fig. 4C).15 When particle precursor chemistry is well-controlled, the solution phase synthesis can lead to large-scale preparation of Fe3O4 NPs with NP sizes tuneable in 1 nm increments as highlighted by 12 nm Fe3O4 NPs (Fig. 4D) synthesized through the thermal decomposition of a Fe–oleate complex.16
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Fig. 3 General schematic of NP synthesis starting from the fast formation of nuclei and subsequent growth with common synthetic parameters which can be used to control NP sizes and shapes. Reprinted from ref. 9a with permission, Copyright 2006 John Wiley & Sons, Inc.

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Fig. 4 TEM images of (A) 5 nm Au NPs (reprinted from ref. 13b, Copyright 2008 Tsinghua Press and Springer-Verlag GmbH), (B) 10 nm Co NPs (reprinted from ref. 8c with permission, Copyright 2015 American Chemical Society), (C) 4 nm Fe NPs with controlled Fe3O4 shells (reprinted from ref. 15 with permission, Copyright 2006 American Chemical Society), (D) 12 nm Fe3O4 NPs synthesized through metal oleate decomposition (reprinted from ref. 16 with permission, Copyright 2004 Springer Nature), (E) alloy FePt NPs (reprinted from ref. 17b with permission, Copyright 2004 American Chemical Society), and (F) alloy NiPd NPs (reprinted from ref. 18b with permission, Copyright 2018 John Wiley & Sons, Inc.).

To prepare alloy NPs, the reaction leading to alloy component nucleation and growth must be even more carefully controlled. For example, in the preparation of alloy FePt NPs, the Pt precursor, Pt(acac)2, was reduced and Fe(CO)5 decomposed at about 200 °C to initiate the nucleation of the FePt alloy before the reaction temperature was raised further for FePt NP growth to occur and form 6 nm FePt NPs (Fig. 4E).17 If two metal salt precursors are chosen for the formation of alloy NPs, the correct reducing agent should be used to reduce both metal salts concurrently, as demonstrated in the synthesis of 3 nm NiPd NPs (Fig. 4F) via co-reduction of Pd(acac)2 and Ni(OAc)2 by borane tert-butylamine at 100 °C in an oleylamine solution of 1-octadecene.18 This co-reduction method has been a popular choice for preparing various alloy NPs.19

When the growth of NPs is controlled at one specific crystal facet, edge or corner, different shaped NPs can be prepared. For example, polyhedral Pt NPs can be synthesized through the reduction of Pt(acac)2 and stabilization by oleylamine and oleic acid in benzyl ether.20 However, when a metal carbonyl group or pure CO is present, preferential exposure of Pt {100} facets is achieved during NP growth, giving Pt nanocubes.21 Through control of the CO amount, Pt octahedra (Fig. 5A), icosahedra, and hyper-branched uniform Pt structures could be synthesized.22 Kinetic control over NP synthesis was better demonstrated in the synthesis of Ag and Pd nanocubes in the presence of Cl or Br ions.23 Cl ions acted as a specific capping agent for the {100} facets, which allowed the formation of sharp corners and edges of the nanocube, even at low temperatures (Fig. 5B). Kinetic control to prepare Au nanorods with a uniform geometry and aspect ratios has been established by micelle-templated growth through the surfactant hexadecyltrimethylammonium bromide (CTAB) and other reaction parameters like temperature and surfactant mixtures.24 An additive, like salicylic acid, was proved imperative to increase the yield of Au nanorods (Fig. 5C) through modification of the micelle-formation of CTAB.25 Controlling ratios of CTAB with ascorbic acid was also explored to control Au nanorod growth into more complicated NP structures, as demonstrated in the synthesis of core/shell Au/Pd octopods (Fig. 5D) and concave structures.16 Reduction of Fe(acac)3 and Co(acac)2 in the presence of oleic acid and sodium oleate yielded monodisperse CoxFe3−xO4 nanocubes (Fig. 5E).17 Sodium oleate has been used in many NP syntheses to control the shape and aspect ratio of nanocubes, nanorods, and nanowires.28 Comparable to the formation of Au nanorods, FePt nanorods/nanowires (Fig. 5F) were synthesized through tuning the ratio of oleylamine and 1-octadecene; more oleylamine resulted in longer nanowires.29


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Fig. 5 (A) TEM image of an assembly formed from Pt octahedra (reprinted from ref. 22b with permission, Copyright 2013 American Chemical Society), (B) TEM image of Ag nanocubes (reprinted from ref. 23a with permission, Copyright 2016 American Chemical Society), (C) TEM image of Au nanorods (reprinted from ref. 25 with permission, Copyright 2012 American Chemical Society), (D) scanning electron microscopy (SEM) image of Au/Pd octopods (reprinted from ref. 26 with permission, Copyright 2011 American Chemical Society), (E) SEM image of an assembly of CoxFe3−xO4 nanocubes (reprinted from ref. 27 with permission, Copyright 2014 American Chemical Society), and (F) TEM image of FePt nanowires (reprinted from ref. 29b with permission, Copyright 2013 John Wiley & Sons, Inc.).

Seed-mediated growth differs from the conventional nucleation/growth process in that seeding NPs are pre-made and present in the reaction solution for further growth.30 In this growth process, new nucleation processes should be avoided. The success of this synthesis is dependent primarily on the seed quality and the control of the growth on the seeding NP surface. Compared to the nucleation/growth approach, the seed-mediated growth method is advantageous to control not only NP sizes, but also NP structures and morphologies, such as core/shell NPs with controlled core dimensions and shell thicknesses. In general, seeding monodisperse NPs are present, along with monomers for shell formation; a successful core/shell synthesis involves entering the growth stage on the seeding NP without individual nucleation of the shell materials. This strategy has been used to create both monodisperse Pd/Au (Fig. 6A)31 and Au/Pd (Fig. 6B)32 core/shell NPs, among many other bimetallic core/shell structures.33 With appropriate synthetic control, a more complicated alloy shell can also be made, as demonstrated in the synthesis of Ni/FePt NPs (Fig. 6C).34 Similarly, more shells can be grown in a core/shell structure through multiple growth steps, as seen in the synthesis of Pd/Au/FePt NPs (Fig. 6D).31


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Fig. 6 (A) STEM image of Pd/Au core/shell NPs, with a high-resolution TEM image shown in the inset (reprinted from ref. 31 with permission, Copyright 2010 John Wiley & Sons, Inc.), (B) TEM image of Au/Pd core/shell NPs, with a high-resolution TEM image shown in the inset (reprinted from ref. 32 with permission, Copyright 2012 John Wiley & Sons, Inc.), (C) TEM image of the assembly of Ni/FePt core/shell NPs, with an HAADF-STEM image of one NP shown in the inset (reprinted from ref. 34 with permission, Copyright 2014 American Chemical Society), and (D) TEM image of Pd/Au/FePt core/shell NPs, with line-scan elemental analysis demonstrating the presence of two shells shown in the inset (reprinted from ref. 31 with permission, Copyright 2010 John Wiley & Sons, Inc.).

The seed-mediated growth method can be extended to prepare more complicated heterostructures to contain multiple components.35 For example, bifunctional Au–Fe3O4 dumbbell NPs (Fig. 7A) could be synthesized through the controlled nucleation and growth of magnetic Fe3O4 on plasmonic Au NP seeds.36 A similar approach was used to synthesize dual magnet FePt–Fe3O4,37 and the post-modification could access more complex structures such as FePt–Fe2C,38 among others.39 Semiconducting oxides, such as In2O3, can also be grown on the magnetic FePt NP surface (Fig. 7B).40Via similar seed-mediated growth, libraries of heterostructures have been discovered and studied.41Fig. 7C highlights successive heterostructure growth to form a chain of four different NP units.42 To create this structure, Au was grown selectively on the Pt surface of the Pt–Fe3O4 dumbbell NPs. Furthermore, Cu–S was added as the fourth component which preferentially grew on the Au surface. Such a strategy, combined with post-synthetic exchange mechanisms, has been used to create more combinations of complex heterostructures.43 Recently, mechanisms of the crystallographic attachment and the assembly of heterostructures have also been explored.44 In Fig. 7D, controlled growth of Au on CdSe–CdS QDs was made possible by the seed-mediated approach to create “patchy” heterostructures and further used to study controlled superlattice formation of heterostructures.45


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Fig. 7 (A) TEM image of Au-Fe3O4 dumbbell NPs, with a high-resolution TEM image shown in the inset (reprinted from ref. 36 with permission, Copyright 2010 American Chemical Society), (B) STEM image of FePt-In2O3 dumbbell NPs, with a high resolution TEM image shown in the inset (reprinted from ref. 40 with permission, Copyright 2011 American Chemical Society), (C) TEM image of CuxS-Au–Pt-Fe3O4 heterotetramer NPs, with a high resolution TEM image of the individual components shown in the inset (reprinted from ref. 42 with permission, Copyright 2011 Springer Nature), and (D) TEM image of as-synthesized “patchy” Au-QD heterostructures, with a high-resolution TEM image shown in the inset (reprinted from ref. 45 with permission, Copyright 2019 American Chemical Society).

Monodisperse NPs for catalysis

General catalytic principles

The ever-increasing societal demands for energy consumption and industrial chemical production have triggered great efforts in seeking renewable and environmentally friendly energy/chemical conversion processes, such as in fuel cells and batteries, and CO2 or biomass conversion into valorized chemicals or fuels. The need to limit fossil fuel use and the fast-developing renewable energy and chemical industries require fundamental studies and efficient screenings of highly active and durable catalysts.46 Recently, monodisperse NP catalysts for catalytic reactions for renewable energy/chemical production including the oxygen reduction/oxidation reaction (ORR/OER),47 hydrogen evolution/oxidation reaction (HER/HOR),48 formic acid/alcohol oxidation reaction (FAOR/AOR),49 CO2 conversion and biomass conversion, have been extensively studied. Some emerging catalytic reactions, namely hydrogen peroxide production via the ORR50 and nitrogen reduction reaction (NRR),51 have also attracted a lot of research interest.

A good catalyst should have a balanced binding energy to the reactants, intermediates and products involved in a catalytic process so that the reactants can be strongly adsorbed and activated, meanwhile the product should bind weakly and should be easily desorbed from the catalyst surface. The concept of an optimal catalyst is well illustrated in the Sabatier principle, as shown in Fig. 8A, where the desired catalysts sit at the peak of the “volcano plot” referred to as a “hot spot” for catalysis.52


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Fig. 8 (A) Schematic of the Sabatier principle for catalysis in general, (B) a calculated free energy diagram for the HER at a potential U = 0 relative to the standard hydrogen electrode at pH = 0 (reprinted from ref. 53 with permission, Copyright 2005 American Chemical Society), (C) a volcano plot of the measured exchange current density plotted versus the calculated free energy of H adsorption at U = 0 V. The metals on the left side of the volcano have high H coverage (1 ML) and the metals on the right side have low H coverage (0.25 ML). The line is a prediction by a kinetic model in which all input parameters are taken from DFT calculations. The dashed line indicates that the metals which bind H stronger than 0.2 eV/H usually form oxides at U = 0 V. The open circles are (111) facet data whereas the filled circles are polycrystalline data (reprinted from ref. 54 with permission, Copyright 2010 American Chemical Society), and (D) calculated the limiting potential (the potential where an overall electrocatalytic reaction becomes endergonic) for the ORR (reprinted from ref. 56b with permission, Copyright 2015 Oxford University Press).

In the case of reactions where only one intermediate is involved (e.g. HER/HOR) the volcano plot provides a simple yet comprehensive model for understanding the fundamentals of catalysis. An active catalyst is required to have a near-equilibrium binding energy to H* (* stands for a catalytically active site that binds to the adsorbate), as understood from the density functional theory (DFT) calculations (Fig. 8B).53 Platinum group metals (PGM), including Pt, Re, Pd, Rh and Ir, are the best monometallic catalysts for the HER as they are adjacent to the apex of the volcano plot (Fig. 8C), where ΔG is the calculated Gibbs free energy of H* adsorption at potential U = 0 V.54 H* adsorption is the only but an effective descriptor for predicting catalyst activity towards the HER, and thus the free energy diagram correlates well with the volcano plot. As a simple and straightforward guideline for discovering new catalysts, this model works effectively in explaining the experimental results and predicting the trend of catalyst activity, especially in the case of the HER, yet it disregards the factors of adsorbate coverage, solvent effects and kinetics. In the overall hydrogen catalysis, H2 ↔ 2H+ + 2e, Pt reversibly catalyzes both the HER and HOR at a negligible overpotential under acidic conditions. In contrast, the HER/HOR catalysis on Pt under alkaline conditions requires a more significant overpotential.55 The volcano plot successfully explains why Pt is superior to other catalysts for hydrogen catalysis but fails to give any insights into the different performances under different pH conditions. The “optimal catalyst” is at the apex of a volcano plot which is a hypothetical point. Experimentally, the best catalyst may not sit at the apex due to certain limits in preparing a catalyst surface.

The volcano plot with a single descriptor is not sufficiently effective at determining the trends of catalytic activity and selectivity for catalysis involving multiple intermediates. Although a catalyst surface can be tuned towards an optimal adsorption energy for one specific intermediate that is involved in a rate-determining step, its binding strength to related intermediates is strongly correlated and is unable to be de-coupled due to the “scaling relationship”. In the case of ORR catalysis on a Pt surface, OOH* adsorption energy has a nearly linear relationship with OH* adsorption energy on different facets of Pt and Pt-based alloy catalysts, as shown in Fig. 8D. OOH* binds weakly to the Pt surface and OH* binds strongly to the Pt surface. Ideally, it is most favored to enhance the binding of OOH* and simultaneously weaken the binding of OH*, but this is forbidden on a single-component catalyst surface due to the strongly coupled scaling relationship. Finding ways to break the scaling relationship between multiple intermediates on a catalyst surface is a cutting-edge research frontier. Strategies such as creating strains to favor different adsorbates at different catalytic sites,56 introducing a second catalytic site to the primary catalytic site57 and using a combination of cascade/tandem catalysts to favor the adsorption of various adsorbates onto different catalytic sites, have been recently developed for different catalytic reactions.58 In those cases, the basic concept is to have one intermediate generated at the first catalytic site, followed by the “spillover” of the intermediate to a second catalytic site which favors different steps of the reaction.

The development of efficient catalysts can be realized by maximizing the catalytically active sites and/or enhancing the intrinsic activity of a catalyst surface. With well-defined size, morphology, shape/facet control and component stoichiometry, monodisperse NPs are ideal subjects for studying and identifying active sites for catalysis.

Fuel cell electrocatalysis

Oxygen reduction reaction (ORR) catalysis has been one of the most intensively studied topics because it is the rate-determining step which limits the overall energy conversion efficiency of fuel cells and batteries.59 The state-of-the-art single-component catalyst for the ORR is Pt. However, Pt binds to oxygenated species (O*) strongly compared to its optimal value in the volcano plot, as shown in Fig. 9A.60 Monodisperse Pt NPs were synthesized and tested for the ORR. In HClO4 solution, it was found that Pt NPs of about ∼2.2 nm had the highest mass activity (activity normalized to the Pt weight) and specific activity (activity normalized to the surface area).61 The facets exposed at the surface of NPs are also paramount; the Pt (111) facet is the most active facet in HClO4 solution towards the ORR, while (110) and (100) facets are much more active in H2SO4 solution.62 This acid-induced catalysis change on different crystal facets is attributed to the anion interaction with the Pt surface. For example, in a strongly adsorbing electrolyte, such as sulfuric acid, Pt (100) and (110) facets are relatively more active due to the strong tridentate bond of SO42− to the (111) facet.63 An alternative strategy is to alloy Pt with another non-noble metal to introduce electronic (ligand), strain (geometric) and ensemble (coordination) effects.64 All of these fundamental studies require to have monodisperse NPs as the catalyst so that a catalytically active/selective surface can be better identified for catalysis optimization.
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Fig. 9 (A) Trends in oxygen reduction activity plotted as a function of the O* adsorption energy (reprinted from ref. 60 with permission, Copyright 2004 American Chemical Society), (B) schematic representation of the PtNi nano-octahedra with different composition, morphology and surface structure changes after electrochemical surface activation (25 potential cycles) and electrochemical stability tests relative to as-synthesized NPs, (C) an STEM image of PtNi1.5 concave octahedra after 25 potential cycles, suggesting leaching of the Ni component (B and C are both reprinted from ref. 65a with permission, Copyright 2013 Springer Nature), (D) Mo73Ni1143Pt3357 NPs at 170 °C as determined by Monte Carlo simulation. Occupancies are indicated by the color triangle on the right. Small spheres represent the atoms in the outer layer (reprinted from ref. 66 with permission, Copyright 2015 The American Association for the Advancement of Science). (E) Schematic illustrations and corresponding TEM images of the samples obtained at four representative stages during the evolution process from polyhedral to nanoframes (reprinted from ref. 67 with permission, Copyright 2014 The American Association for the Advancement of Science).

An intensively studied system is octahedral PtNi NPs with the surface being exclusively the (111) facet, which has been demonstrated as very active for the ORR in HClO4 (Fig. 9B).65 However, Ni in the NiPt structure is not stable, leaching out preferentially near the center of the (111) facet during ORR catalysis under acidic conditions, and as a result, the shape of PtNix may evolve into concave structures as illustrated in Fig. 9C and further into multipods. Pt-rich Pt1.5Ni better maintained the (111) facet after the electrochemical leaching of Ni compared to Ni-rich PtNi1.5, which contributed to increased activity in the etched structure. The structural evolution of the shape-controlled NPs complicates ORR catalysis, suggesting the importance of maintaining the shape and the catalytic components.

To further enhance the ORR catalysis, transition-metal doping strategies were applied to the PtNi octahedra.66 The doping was accomplished by seed-mediated growth of the transition-metal on the PtNi octahedra with metal carbonyl precursors. Cr-, Fe-, Co-, Mo-, Re- and W-doped Pt3Ni octahedra showed superior mass activity to the seed NPs. Mo-doped Pt3Ni demonstrated a specific activity of 10.3 mA cm−2 and a mass activity of 6.98 A mgPt−1 in O2-saturated 0.1 M HClO4 at room temperature. A modeling study showed that Mo has a strong driving force to segregate at the surface, preferentially occupying the vertices and edges connecting adjacent (111) facets (Fig. 9D). The surface-segregated Mo stabilizes Pt and Ni against dissolution, thus enhancing the stability of PtNi octahedra under ORR conditions. In another case, structural evolution of shape-controlled PtNi NPs was found to lead to a three-dimensional porous structure which is highly active towards the ORR.67 PtNi3 rhombic dodecahedra NPs were synthesized in oleylamine and slowly transformed into porous nanoframes, as shown in Fig. 9E. The structural evolution from electrochemical leaching was also found in the ultrathin PtNi nanowires. The core/shell Pt/NiO nanowires slowly transformed into rough-surfaced Pt nanowires with a trace amount of Ni left in the core.68 The de-alloyed PtNi nanowires showed an unprecedentedly high activity towards the ORR in 0.1 M HClO4 at room temperature. The modeling study showed that the de-alloyed PtNi nanowires possess a large portion of the undercoordinated surface and the Pt–Pt bond is compressed, resulting in ensembles similar to the Pt (111) facet.

Despite their impressive ORR activity demonstrated in 0.1 M HClO4 at room temperature, the proton exchange membrane fuel cells operate at 80 °C and the catalysis occurs at a solid–liquid–gas three-boundary interface which is not often studied and those tested under the membrane electrode assembly (MEA) conditions did not show much impressive and consistent catalysis enhancement compared to commercial Pt. The best fuel cell performance was demonstrated on de-alloyed ∼5 nm PtNi NPs showing improved durability.69 However, the acid-pretreated PtNi NPs still suffered from significant loss of the Ni component after 30[thin space (1/6-em)]000 cycles in a fuel cell test with only ∼4–15% Ni remaining.

Intermetallic NPs, specifically L10-structured (tetragonal) NPs, have provided an effective approach to stabilize Pt-based alloys under corrosive fuel cell conditions.70 9 nm MPt (M = Fe, Co) with an L10-structured core and a compressively strained Pt shell with 2–3 atomic layers (Fig. 10A and B) was found to effectively stabilize M under an MEA condition of 80 °C.71 The intermetallic NP catalyst showed high activity before and after catalysis, respectively, as seen in Fig. 10C, while maintaining the composition of M at ∼40% after 30[thin space (1/6-em)]000 cycles of a durability test across the MEA electrode which was imaged after the durability test (Fig. 10D). This activity and stability beat the DOE 2020 targets of 0.44 A mgPt−1 and less than 40% loss after 30[thin space (1/6-em)]000 cycles in mass activity.72 Due to smaller lattice constants of L10–CoPt compared to those of L10–FePt, the Pt shell in the L10–CoPt/Pt structure is compressed further (−4.50%/−4.25% biaxial strain), binding more weakly to oxygenated species and thus exhibits higher ORR activity. PtPb nanoplates were also synthesized to form an intermetallic structured core (hexagonal) surrounded by a Pt shell, showing an excellent mass activity of 4.3 A mgPt−1.73 It also demonstrated very stable ORR performance up to 50[thin space (1/6-em)]000 cycles in 0.1 M HClO4 at room temperature; yet Pb still suffered from the leaching problem (from ∼48% to ∼15%) in the structure after the durability test.


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Fig. 10 (A) TEM image of the C–CoPt NPs after annealing to form a spherically shaped NP, (B) HAADF-STEM image of core/shell L10-CoPt/Pt NPs with 2–3 atomic layers of the Pt shell over the L10-CoPt core before the durability test, (C) mass activities of tested L10-CoPt via the current DOE protocol compared with activity targets of the DOE, (D) elemental mapping of a large-area of the MEA assembled with L10-CoPt/Pt NPs after the durability test in MEA at 80 °C (A–D are all reprinted from ref. 71 with permission, Copyright 2019 Elsevier), and (E) free-energy diagram of the ORR pathways. The proposed associative reaction coordinates represent the following states: (I) * or # + O2 + 4H+ + 4e, (II) OOH* or OOH# + 3H+ + 3e, (III) O* or O# + H2O + 2H+ + 2e, (IV) * + H2O2 + 2H+ + 2e, (V) 2OH# + 2H+ + 2e, (VI) OH* or OH# + H2O + H+ + e, and (VII) * or # + 2H2O, where * (blue) denotes the binding site on Co-N4 embedded in graphene and # (gray) denotes the binding site on a strained Pt (111) facet. (Inset) Schematics of H2O2 generated over Co-N4 migrating to the strained Pt (111) surface (green arrows), followed by dissociation to OH# and water formation (reprinted from ref. 74 with permission, Copyright 2018 The American Association for the Advancement of Science).

Using the concept of cascade catalysis, CoPt NPs were incorporated into another catalytic site, a Co–N–C material, which was derived from a Co- and Zn-containing metal organic framework (MOF).74 The MOF was first synthesized and subsequently loaded with a Pt precursor. The Co component from the MOF serves as the precursor to alloy with Pt during high-temperature reaction to form CoPt NPs. This catalyst achieved a mass activity of 1.77 A mgPt−1. The DFT calculations showed that the Co–N4 catalytic site favors the 2 electron-pathway by forming H2O2 with a relatively low overpotential (Fig. 10E). The generated H2O2 can easily desorb and migrate to adjacent CoPt NPs, subsequently being reduced to H2O by PtCo via a thermodynamically spontaneous step. The synergistic catalysis can thus bypass the strongly bonded O* and OH* intermediates on the Pt surface, resulting in an overall enhanced ORR catalysis. However, assigning all catalytic activity to Pt may underestimate the catalytic performance when calculating the mass activity as the Co–C–N catalytic sites also showed activity in fuel cells.75

Novel NP catalysts have greatly advanced the ORR catalysis and fuel cell applications. Nevertheless, a huge gap exists between the screening process of the ORR catalysts in either an HClO4 or H2SO4 electrolyte via the liquid half-cell test and the device applications via the MEA test. Some representative catalytic systems are summarized in Fig. 11, illustrating also the commercial Pt NPs supported on carbon.64 The discrepancy under different testing conditions implies that the current rotating disk electrode (RDE)-based liquid half-cell test may overestimate the ORR activity of Pt-based alloy NPs. The possible reasons for this discrepancy are as follows: (1) oxygen diffusion and water product removal are a common issue at the three-boundary interface of the catalyst layers in MEA, while in the thin film working electrode tested in a liquid half-cell, oxygen can be easily accessible and products are efficiently removed via rotation; (2) the catalyst layer uses a perfluorosulfonic acid ionomer as the electrolyte, but the electrolyte used in these RDE tests shown in Fig. 11 is HClO4, which shows negligible bonding to the Pt surface and thus the Pt surface is more accessible to reactants and intermediates; and (3) the MEA condition of 80 °C is harsher than that of the RDE test performed at room temperature, resulting in possible leaching of non-precious components in Pt-based alloy NPs and possible shape evolution which makes them behave more like pure Pt catalysts with no structural control. As the MEA test is time-consuming and expensive, more fundamental studies on an appropriate and rapid screening test should be conducted to understand this performance gap. For the synthetic control of NP catalysts, robust NPs with preferred surfaces should be further developed. To better understand the ORR catalysis of NPs, as well as other relevant fuel cell reactions, the structure, morphology and composition of NPs should be assessed carefully with advanced in situ and ex situ techniques during the reaction.


image file: c9nr06080d-f11.tif
Fig. 11 ORR mass activity of representative NP catalytic systems measured in a liquid half-cell at room temperature and in MEA at 80 °C. The data points and corresponding TEM and HAADF-STEM images are reprinted from ref. 65a (with permission, Copyright 2013 Springer Nature) and ref. 69 (published by The Royal Society of Chemistry) for PtNi de-alloyed NPs, ref. 67 (with permission, Copyright 2014 The American Association for the Advancement of Science) for PtNi nanoframes, ref. 73 (with permission, Copyright 2016 The American Association for the Advancement of Science) for PtPb nanoplates, ref. 68 (with permission, Copyright 2016 The American Association for the Advancement of Science) for PtNi de-alloyed nanowires and ref. 71 (with permission Copyright 2019 Elsevier) for L10-CoPt NPs.

Electrochemical reduction of CO2

The selective conversion of CO2 to an active form of carbon has been one of the most widely studied problems in the past decade and the use of monodisperse NP catalysts has come about as one of the possible solutions to increase reaction activity and selectivity. In 1985, Hori used Cu as the catalyst to electrochemically reduce CO2 to hydrocarbons such as methane and ethylene in one-step under ambient conditions (room-temperature and pressure) in aqueous solutions.76 In this method, electricity serves as the energy source, with the possibility to be generated by renewable sources such as wind and solar energy, and H2O and CO2 act as the renewable hydrogen and carbon sources, which is ideal for greener CO2 conversion processing.77 The fast development of NP synthesis has made it possible to study in more detail the CO2 reduction reaction (CO2RR) mechanism,78 especially for the CO2RR yielding CO on a Au surface.79 To reveal what sites on the catalyst surface are active and selective for the CO2RR yielding CO, stable monodisperse Au NPs were prepared and studied.80 Monodisperse Au NPs with diameters of 4, 6, 8 and 10 nm were prepared through the burst nucleation method (using a strong reducing agent). Among Au NP catalysts with different sizes, 8 nm Au NPs showed the best CO2 reduction activity for the formation of CO; the faradaic efficiency (FE) reached 90% at −0.67 V vs. RHE (Fig. 12A and B). Density functional theory (DFT) calculations highlighted that 8 nm Au NPs show the best activity because their crystal domain has the largest edge/corner ratio and the edge site is the most active site for CO formation. With the observation and calculation from monodisperse Au NPs of this favoured facet configuration, the work was extended to the synthesis of monodisperse 2 nm ultrathin Au nanowires of different lengths as seen in Fig. 12C.81 This synthesis was chosen due to the abundance of edge sites on the ultrathin Au nanowires. After electrochemical CO2 reduction under the same conditions (0.1 KHCO3 aqueous solution and CO2 bubbling), the 500 nm long ultrathin Au nanowires produced CO with a FE of 94% at −0.35 V vs. RHE, which shows a higher FE and lower overpotential than 8 nm Au NPs, seen clearly in Fig. 12D. The increased activity and selectivity were explained by the larger edge/corner ratio in the longest ultrathin nanowires compared with the shorter nanowires and also the NPs. Using monodisperse NPs to investigate the active site of CO2 electroreduction and further improve their activity has also been pursued on other NPs, such as Ag and Pd, and this demonstrates an exciting direction in the optimization of NP catalysts for the CO2RR.82
image file: c9nr06080d-f12.tif
Fig. 12 (A) TEM image of monodisperse 8 nm Au NPs, (B) the reduction potential-dependent faradaic efficiency (FE) of CO formation from CO2 electroreduction over Au NP catalysts (A and B reprinted from ref. 80 with permission, Copyright 2013 American Chemical Society), (C) TEM image of 500 × 2 nm Au nanowires, and (D) the reduction potential dependent FE for CO formation from CO2 electroreduction in 0.5 M KHCO3 over different lengths of Au nanowires (C and D reprinted from ref. 81 with permission, Copyright 2014 American Chemical Society).

The core/shell architecture and control of the NP composition have also been applied to create new catalysts for the CO2RR. The effect of the inner core was observed to affect the overall reaction selectivity and activity, as demonstrated in the Cu/SnO2 core/shell NPs with 0.8 nm or 1.8 nm SnO2 shells as catalysts for CO2 electroreduction (Fig. 13A–E).33b Even though the thickness of the SnO2 shell only changes 1 nm from 0.8 nm to 1.8 nm, the selectivity of the CO2 reduction products is changed from CO to formate. DFT calculations suggested that a trace amount of Cu could diffuse into the 0.8 nm SnO2 shell and affect the lattice strain of the SnO2 shell. A similar effect was observed through the construction of monodisperse Cu/In2O3 core/shell NPs, but the focus was on tuning CO formation through altering the In2O3 shell and CO2RR reduction potentials.83 Such a core/shell architecture was further extended to other systems, such as a Ag/Sn bimetallic catalyst for selective formation of formates.84


image file: c9nr06080d-f13.tif
Fig. 13 (A) TEM image of 7/0.8 nm core/shell Cu/SnO2 NPs, (B) EELS elemental mapping on one 7/0.8 nm Cu/SnO2 NP, and (C) EELS line scan of one 7/0.8 nm Cu/SnO2 NP. Electrochemical CO2 reduction results for product formation of CO, formate, and H2 of (D) C-7/0.8 nm Cu/SnO2 NPs and (E) 7/1.8 nm Cu/SnO2 NPs. Images reprinted from ref. 33b with permission, Copyright 2017 American Chemical Society.

Compared with stable noble metal NP catalysts, Cu catalysts have shown great promise as a means to access hydrocarbons. However, pure Cu NPs are unstable and poorly selective.85 To stabilize Cu, monodisperse AuCu NPs were prepared and studied for the CO2RR yielding CO.86 Alloying Cu with Pd yielded comparable selectivity to CO as did alloying with Au.87 Beyond alloying, 7 nm monodisperse Cu NPs assembled on pyridinic-N rich graphene (p-NG) showed reduction potential dependent selectivity to formate at −0.8 V, but to C2H4 at −0.9 V or beyond.88 Because of the ability of p-NG to act as a CO2 and proton adsorber, combined with synergistic Cu activity for hydrogenation and C–C coupling, the composite structure was much more active and also stable compared to those of pure Cu NPs for CO2 electroreduction. Recently, the shape within monodisperse Cu NP catalysts was explored to tune the CO2RR. Cu NWs have been synthesized through various methods and have been able to change the hydrocarbon selectivity of Cu based on the morphological features of the NWs.89 For example, when ∼20 nm diameter Cu NWs were tested for the CO2RR, 55% FE for methane production was achieved, and this FE was shown to change throughout the reaction, indicating the beginning of the formation of a notable amount of ethylene, due to the morphology change of the NWs with the potential. When the Cu NWs are 50 nm in diameter with a larger fraction of (100) facets exposed, a FE of 60% for C2 hydrocarbons (C2H4 and C2H6) was achieved from CO reduction. CO2RR selectivity on these NWs was lower due to the CO2RR yielding CO required for C2-product formation. One new strategy has been to couple two different metals together in cascade/tandem catalysts, one which has selectivity toward forming CO and the other which can transform CO to hydrocarbons, and until now the latter has been Cu.90 An example of this has been studied on coupling Au and Cu, to act as a bifunctional catalyst to improve the FE for different kinds of hydrocarbons.90c

Green chemistry applications of monodisperse NPs

Green chemistry refers to the reduction of hazards and waste associated with chemical synthesis and applications. The twelve principles of green chemistry were first established to offer guidelines on what it means for a chemical, a process, or a procedure to be “green”.91 Within these twelve principles there are countless new research directions, many centred around one of the twelve principles: catalysis. To make a chemical conversion more sustainable, catalytic reactions are preferred compared to those conversions with stoichiometric reagents and chemical additives.

Using NPs as catalysts for chemical conversions can be more advantageous than homogeneous catalysts. Supported NP catalysts can easily be separated from the reaction mixture through filtration, centrifugation, or coupling with a magnetically separable component allowing for easy catalyst reuse and isolation of the products of the reaction.92 Furthermore, with advances in NP synthesis, catalysts can be optimized either through adding a bifunctional component to the catalyst or intrinsically changing the activity of the active component.

Two of the primary principles of conducting green chemistry are enabling less hazardous chemical syntheses and creating inherently safer conditions, both to the researcher and to the environment. When considering a common hydrogenation reaction, conventional approaches utilize high-pressure hydrogen (H2) cylinders as a source of H2 for the reaction often combined with high-pressure reaction set-ups. Monodisperse NP catalysed dehydrogenation of small hydrogen storage molecules is considered a green chemistry alternative to hydrogenation for on-site or in situ formation of H2.

Of the H2 sources considered, ammonia borane (AB) is particularly interesting and widely studied because of its high weight percent of hydrogen (19.5%) and stability under standard conditions.93 Furthermore, it is not flammable or toxic, making it a possible green alternative to other reductants such as sodium borohydride (NaBH4) or Lindlar's catalyst which employs lead or quinoline as a catalyst poison.94 AB can undergo catalytic hydrolysis or methanolysis to release 3 mol H2 for every 1 mol of AB. Monometallic metal NP catalysts (Pt, Pd, Ru, Cu, and Ni) have been developed to release H2 from AB through AB hydrolysis or methanolysis,95 and such progress has been reviewed recently.96 In an effort to both stabilize NPs against leaching and increase the activity of NP catalysts, there have been many studies trying to develop bimetallic and even trimetallic NP systems to optimize AB dehydrogenation (CoPd, FePd, PtPd, CuPt, CoPt, and NiPt).97 Alloying non-noble metals such as Cu and Ni in 16 nm monodisperse CuNi NPs, or others, has also improved the catalyst stability and activity.98 Efforts have also been made towards immobilizing monodisperse NPs on robust supports to further combat stability issues. For example, Ni NPs supported on molybdenum disulphide (MoS2) were demonstrated to effectively produce H2 from AB, while also being very stable.99

In addition to AB, formic acid (FA) is also a promising H2-storage molecule. FA, a product of biomass decomposition, undergoes catalytic decomposition to form gaseous products (CO2 and H2via a direct dehydrogenation pathway, and sometimes CO via a dehydration pathway).100 Monodisperse bimetallic Pd-based NP alloys such as 3.8 nm AuPd and 2.2 nm AgPd were prepared and found to increase the activity of NPs for the decomposition of FA under mild conditions (1 atm, 50 °C), without the formation of CO.101 Interestingly, surface control of AgPd NPs to create alloys with various surface exposures of Pd atoms has been used as a strategy to achieve optimized CO2 hydrogenation to form FA under moderate conditions (20 atm, 100 °C), highlighting a potentially reversible process in which FA can be a hydrogen storage material.102 Similar to AB dehydrogenation, support interactions have been utilized to increase the activity of FA dehydrogenation. 3.3 nm Au NPs supported on Al2O3 and 1.8 nm Au NPs supported on ZrO2 have also been shown to decompose FA under mild conditions, with the primary source of stability and activity coming from the metal–support interactions.103 Pd coupled with pyridinic-N-doped carbon, NiPd or AuPd on NH2-functionalized and N-doped reduced graphene oxide, and AgPd coupled with oxygen-deficient tungsten oxide (WO2.72), among others have all been utilized to maximize activity and stability.104 The creation of NPs with a core–shell architecture has also proved to be valuable for improving FA decomposition.105 For example, 7.3 nm monodisperse core/shell Au/Pd NPs boosted catalytic activity due to localized surface plasmon resonance (LSPR) effects of Au on Pd.105b

The NP catalysts active for H2 generation from AB or FA are often active for catalysing hydrogenation reactions under mild conditions, making these NPs an attractive class of new catalysts for tandem reactions. In the past decade, tandem catalysis (also referred to as domino catalysis or one-pot catalytic reactions) has been gaining popularity in organic syntheses to minimize reactants, solvents, and potentially even catalyst waste. The theory has been reviewed recently, focusing on optimizing multiple reaction conditions.106 Tandem catalysis for the one-pot production of H2 from AB over 3.3 nm monodisperse NiPd NPs and hydrogenation of nitro/nitrile compounds was demonstrated at room temperature and ambient pressure.18a Such tandem reactions could proceed well on many Pd-based catalysts such as shape-controlled AuPd nanorods (AB hydrolysis/4-nitrophenol reduction), CoPd NPs (NaBH4 hydrolysis/nitro group reduction), and CoPd NPs (AB hydrolysis/nitro, nitrile, carbonyl group reduction), among others.107

Beyond nitro/nitrile reduction, tandem NP catalysis has been extended to prepare N-heterocyclic rings such as benzoxazoles and quinazolines under greener chemical conditions. One-pot reactions of FA, 2-nitrophenol, and aldehydes formed benzoxazoles with near quantitative yields over the 2.2 nm monodisperse AgPd NPs (Fig. 14A) coupled with oxygen-deficient tungsten oxide (Ag48Pd52/WO2.72).104a The NPs after coupling can be seen in Fig. 14B, along with a cartoon schematic (Fig. 14C). The control of the NP composition was achieved through co-reduction of the silver and palladium precursors in the presence of oleylamine and a reducing agent; the assembly with WO2.72 was achieved through the growth of AgPd in the presence of preformed WO2.72. Monodisperse NP synthesis and assembly on the WO2.72 nanorods were optimized to obtain a universal catalyst to use H2 formed in situ from FA or ammonium formate (AF) to form a library of value-added N-heterocyclic products under mild conditions, 1 atm, 50–80 °C (Fig. 14D and E). WO2.72 was also used to support monodisperse Cu NPs and improve their catalytic activity for AB dehydrogenation and stability for tandem selective hydrogenation of nitrostyrene.108


image file: c9nr06080d-f14.tif
Fig. 14 (A) TEM image of 2.2 nm Ag48Pd52 NPs, (B) TEM image of the assembly of AgPd NPs on oxygen-deficient tungsten oxide (WO2.72), (C) cartoon schematic of the AgPd/WO2.72, composite structure, (D) catalytic results for the tandem dehydrogenation of FA and formation of benzoxazoles, and (E) catalytic results for the tandem dehydrogenation of ammonium formate and formation of quinazolines. Figures reprinted from ref. 104a with permission, Copyright 2017 American Chemical Society.

Considering the future of tandem catalysis, stabilization of NPs is paramount as the reaction conditions of each part may be very different, and the catalyst itself must be robust to perform universally well while being reusable. Zeolite-matrix and polymer stabilized monodisperse NPs have risen as possible materials to add to NP stability.109 Furthermore, another advantage of monodisperse NPs is their ability to create uniform monolayer assemblies110 which can maximize the surface available for catalysis.111 In a recent study, 3 nm monodisperse NiPd NPs were prepared and assembled on a solid substrate via the transfer of a monolayer assembly (Fig. 15A–C).18b Combined with pre-deposition of a monolayer of nitrogen-doped graphene (NG), a composite structure containing a monolayer of NiPd NPs on a monolayer of NG could be fabricated on a silica or glass substrate. This monolayer composite can serve as a catalyst probe, actively controlling reaction processes and minimizing the work-up procedures for product separation as demonstrated in one-pot hydrolysis of AB, nitro-reduction, and quinazoline synthesis under mild conditions (1 atm and 60 °C) (Fig. 15D); the reaction set-up in Fig. 15E highlights the production of H2 in a balloon which would transfer to the following hydrogenation and other ring-closure reactions for the formation of quinazolines. The catalyst probe was stable for the tandem reactions, showing no obvious activity drop in 5 rounds of reaction tests (Fig. 15F). Such an assembly approach highlights a potential future direction in green chemistry applications of NP catalysts with maximum NP surface exposure for catalytic enhancement and optimization.112


image file: c9nr06080d-f15.tif
Fig. 15 (A) TEM image of a monolayer of NiPd on monolayer nitrogen-doped graphene, (B) atomic force microscopy (AFM) image of monolayer NiPd on monolayer nitrogen-doped graphene on a silica substrate, (C) a general schematic highlighting the assembly of a catalyst probe which is easily reusable and can be removed from the catalytic reaction, (D) catalytic results for the dehydrogenation of AB and tandem formation of quinazolines, (E) a reaction set-up where a ballon captures H2 formed in situ, and (F) catalyst activity for the formation of quinazoline after being recycled. Figures reprinted from ref. 18b with permission, Copyright 2018 John Wiley & Sons, Inc.

Monodisperse NPs also show great potential as catalysts in biomass conversion. General NP catalysts for this application have been reviewed recently.113 Still, the development of catalysts to transform these platform chemicals to value-added chemicals is an ongoing research field. Levulinic acid (LA) and its ester derivatives, furfural, and FA are all platform chemicals which can arise from biomass decomposition.114 Much work has been done to convert LA to gamma-valerolactone (GVL),115 or similarly to pyrrolidones.116 Recently, AuPd and supported Pt NP catalysts have demonstrated successful conversion of LA to libraries of pyrrolidones under the mild reaction conditions at 85 °C and 1 atm H2. 3.8 nm monodisperse alloy Au66Pd34 NPs (Fig. 16A–C) were synthesized through co-reduction.19c The AuPd alloys showed much better activity and stability for the tandem nucleophilic addition and hydrogenation compared to pure Pd or Au NPs for the formation of pyrrolidones (Fig. 16D and E). Coupling Pt NPs with porous titania (p-TiO2) (Fig. 17A–C) is another approach to create a better catalyst for the reductive amination of LA or its esters.117 In both AuPd and Pt–p-TiO2 structures, the catalytically active Pd and Pt sites become “electron deficient” due to Au alloying and strong Pt–TiO2 coupling effects, creating favourable electronic environments to avoid higher pressure and temperature reactions for biomass conversion which have previously been reported.116 Therefore, tuning of interparticle interactions and NP–support interactions is a benefit of optimizing catalysis with monodisperse NPs.


image file: c9nr06080d-f16.tif
Fig. 16 (A) TEM of 3.8 nm Au66Pd34 NPs, (B) high-resolution TEM image of one Au66Pd34 NP, (C) elemental mapping of one Au66Pd34 NP showing alloy distribution of the two components, (D) catalytic activity of different compositions of AuPd alloy NPs for the reductive amination of ethyl levulinate with octylamine under 1 atm H2, and (E) stability measurements for successive runs of the reductive amination of ethyl levulinate with octylamine over the same C–Au66Pd34. Figures are reprinted from ref. 19c with permission from The Royal Society of Chemistry.

image file: c9nr06080d-f17.tif
Fig. 17 (A) SEM image of porous titania nanosheets (p-TiO2), (B) HR-TEM image of 1.8 nm Pt NPs on a p-TiO2 support, and (C) catalytic activity at room temperature and 1 atm H2 pressure for a series of substrates for the reductive amination of LA. Images are reprinted from ref. 117 with permission, Copyright 2019 American Chemical Society.

Beyond these, monodisperse NP catalysts have also been demonstrated to enhance catalysis for other organic reactions,118 including C–C coupling reactions.119

Monodisperse NPs for nanomedicine

Nanomedicine encompasses the applications of nanotechnology to the field of medicine.120 In particular, NPs have been widely investigated for cancer diagnosis and therapy. These applications are feasible because long-circulating NPs can selectively accumulate in tumours via the enhanced permeability and retention (EPR) effect.121 The tumour targeting and retention can be improved by coupling a targeting ligand to NPs. Certain inorganic NPs may function as imaging probes, exploiting their unique magnetic or optical properties. Due to the high surface area, drug molecules can be loaded onto NPs and delivered to tumours. Employing NPs as vehicles may dramatically increase the bioavailability of drug molecules, especially those of poor solubility or stability in systemic circulation. It is possible to achieve NPs with multiple functions which can be used as a theranostic agent for simultaneous drug delivery and imaging. It is even possible to engineer NPs such that payloads can be released in response to an internal or external stimulus.122Fig. 18 illustrates an overview of NP platforms which have been investigated for use in biomedicine. For all of these applications, surface modification of monodisperse NPs is highly important for largely determining the circulation half-lives, drug loading, targeting specificity, and pharmacokinetics of the NPs. Below we review some of our progress in this area, with a focus on magnetic NPs.
image file: c9nr06080d-f18.tif
Fig. 18 Illustration demonstrating how nanomaterials can be modified for use in biomedicine. Depending on the application, various targeting ligands, surface chemistries, sizes, shapes, compositions and physical properties can be optimized to maximize the therapeutic or diagnostic ability of the material. Figure reproduced from ref. 120b, Copyright 2016 MDPI.

Surface modification of NPs

Iron oxide NPs are by far the most studied magnetic NPs for biomedical applications.123 In particular, iron oxide NPs have also been extensively investigated as contrast agents for magnetic resonance imaging (MRI). To take full advantage of their magnetic properties, these NPs should be monodisperse such that each individual NP has nearly identical physical and chemical properties.124 To this end, thermal decomposition is superior to conventional co-precipitation synthesis for providing better size and crystallinity control. However, NPs prepared from thermal decomposition are coated with a layer of surfactants, and they cannot be dispersed in aqueous solutions. A post-synthesis surface modification is often necessary before using NPs for biomedical applications.

Catechol-based surface replacement is a common strategy for iron oxide NP modification. Catechol contains two adjacent hydroxyl groups on the phenol ring which can chelate with transition metals such as Fe with high affinity. Catechol and its analogues, for instance dopamine, can replace surface-bound oleic acid and oleylamine, and by doing so alter the surface properties of the NPs. A series of catechol analogues have been tested for this purpose.125 For optimal colloidal stability, a hydrophilic biomolecule or polymer, such as polyethylene glycol (PEG), is often imparted along with catechol to the particle surface (Fig. 19A). For instance, dopamine was coupled with PEG diacids of different lengths using EDC/NHS chemistry. The resulting ligands can efficiently bind to iron oxide NPs, endowing them with excellent colloidal stability in aqueous solutions.126 In a separate study, dopamine was coupled with methoxy PEG using trichloro-s-triazine (TsT) as a crosslinker.127 TsT is a symmetrical heterocyclic compound containing three acyl-like chlorines with varied reactivities toward nucleophiles such as –OH or –NH2. TsT was first coupled with mPEG2000, and the intermediate was subsequently linked with dopamine. The resulting conjugate was also efficient at rendering iron oxide NPs soluble in water.


image file: c9nr06080d-f19.tif
Fig. 19 Dopamine or other catechol analogues can replace surface-bound oleylamine and/or oleic acid and by doing so, alter the surface properties of iron oxide NPs. This is followed by (A) PEGylation and (B) protein adsorption to make NPs stable in aqueous solutions. Figures reproduced from ref. 124 (with permission of The Royal Society of Chemistry) and ref. 128b (with permission of The Royal Society of Chemistry), respectively.

Macromolecules or polymers may also directly bind to the particle surface. For instance, iron oxide NPs were surface-modified with dopamine, resulting in particles which can be dispersed in polar solvents such as dimethyl sulfoxide (DMSO). When adding these NPs in DMSO into human serum albumin (HSA) solutions in water, the protein molecules were adsorbed onto the particle surface, as shown in Fig. 19B.128 After purification, HSA coated NPs could be collected and redispersed in buffer solutions. This method can be extended to other protein molecules, such as casein, fibrinogen, and avidin.129 Some multidentate polymers, such as polyvinylpyrrolidone (PVP)130 and polyaspartic acid (PASP),131 can be added during particle synthesis, and the resulting iron oxide NPs were readily dispersed in water.

Surface modification not only improves the colloidal stability of NPs in aqueous solutions but also reduces their chances of being opsonized and taken up by the host immune system.132 Indeed, compared to dextran coated iron oxide NPs, those coated with PEG-dopamine showed remarkably reduced uptake by macrophages in vitro.126 Such PEGylation-induced particle protection is well documented and leads to extended blood circulation of NPs.133

NP-based imaging

Magnetic NPs have been extensively studied as T2 contrast agents for MRI. For instance, Feridex, a dextran coated iron oxide formulation, has been used in the clinic for live imaging. The efficiency of T2 reduction, measured from r2 relaxivity, is dependent on the NP size. In general, NPs with smaller sizes have lower magnetization values and smaller r2. Taking PVP coated iron oxide NPs for instance, when the NP size was increased from ∼32 nm to ∼118 nm, the r2 relaxivity was increased from ∼173 to ∼249 mM−1 s−1 on a 7 T magnet.130 Meanwhile, the surface coating may also have an impact on the contrast effects. For instance, when comparing Fe5C2 NPs coated with phospholipid, zwitterion–dopamine–sulfonate (ZDS), and casein coatings,134 it was found that the casein coating led to an r2 enhancement by more than 2-fold. This is attributed to the ability of casein to extend the water diffusion correlation time (τD), which is proportional to r2.135

As mentioned earlier, NP-based tumour imaging often exploits the EPR effect.136 Specifically, tumour blood vessels tend to show abnormal wide gaps and abnormalities which allow for the extravasation of materials with sizes up to several hundred nanometers. This, together with the absence of effective lymphatic drainage, leads to selective accumulation of NPs in tumors.137 For instance, when Fe5C2 NPs were intravenously (i.v.) injected into U87MG tumour-bearing mice, there was decent tumour accumulation at 4 h, manifesting as hypointensities in T2-weighted images.138 This idea can be expanded to multimodality imaging probes. For example, HSA coated NPs could be labelled with both Cy5.5, a near-infrared dye molecule, and 64Cu-DOTA, a radioisotope-bound chelate. The tumour accumulation of the resulting NPs was successfully monitored by three modalities: MRI, fluorescence, and PET, as shown in Fig. 20A–C.128a


image file: c9nr06080d-f20.tif
Fig. 20 Surface-modified magnetic NPs for multi-modality imaging. For instance, HSA coated iron oxide NPs can be coupled both Cy5.5 and DOTA-64Cu. The resulting NPs after intravenous injection accumulated in tumors via the EPR effect. The process can be monitored by (A) fluorescence imaging, (B) PET, and (C) MRI. Figures are reprinted from ref. 128a with permission, Copyright 2010 Elsevier.

To improve tumour targeting beyond the EPR effect, NPs can be coupled with a targeting ligand. For instance, c(RGDyK), a peptide with high affinity towards integrin αvβ3, was conjugated onto PASP coated iron oxide NPs.131 Integrin αvβ3 is a tumour biomarker, often upregulated in tumour endothelial cells and many types of cancer cells.139 When tested in U87MG tumour models, the NPs showed efficient tumour uptake which was mediated by the RGD–integrin interaction. Xie et al. synthesized ultrasmall iron oxide NPs using 4-methylcatechol (4-MC), a catechol analogue, as the surfactant. The resulting NPs could be directly coupled with c(RGDyK) through the Mannich reaction.140 The resulting NPs showed good tumour targeting efficiency and MRI contrast when tested in vivo. Interestingly, unbound NPs were efficiently excreted by renal clearance due to their ultrasmall size (<10 nm in hydrodynamic diameter).

NP-based drug delivery

Surface-modified NPs can be loaded with therapeutics for drug delivery. For instance, Fe3O4 NPs were conjugated with tumstatin, a peptide with antiangiogenic and proapoptotic properties.141 The resultant conjugate was tested in a 3D, multicellular tumour spheroid (MTS) tissue culture model,141 which mimics the tumour environment with leaky endothelium surrounding tumour mass. This NP formulation showed selective targeting and penetration into the endothelium, and had 2 times greater uptake, and 2.7 times greater tumour neo-vascularization inhibition. Additionally, doxorubicin was loaded onto HSA coated iron oxide NPs.142 The NPs after i.v. injection accumulated in 4T1 tumours and released doxorubicin in a sustained manner. This formulation showed a striking tumour suppression effect which was comparable to Doxil and greatly outperformed free doxorubicin.

Drug molecules can also be encapsulated within NPs which have a porous structure. One example is hollow iron oxide NPs.143 These NPs were synthesized by the thermal decomposition of Fe(CO)5, followed by oxidation with trimethylamine N-oxide. This initially yielded iron/iron oxide core/shell NPs, but with further oxidation, produced hollow iron oxide NPs,143 a schematic of which is shown in Fig. 21A. These hollow NPs have ∼2–4 nm pores on the surface through which drug molecules can enter and be encapsulated into the interior. For instance, Cheng et al. successfully loaded cisplatin into the hollow NPs.143 The drug was released in a controlled manner, with a t1/2 of 16 h. The drug loading improved the water solubility of cisplatin, and prevented premature drug degradation. In addition, they conjugated Herceptin, an anti-HER2 antibody, onto the surface of the NPs. The antibody coupling enhanced cancer cell targeting and uptake, reducing the IC50 to 2.9 μM, which far exceeded that of free cisplatin (Fig. 21B).


image file: c9nr06080d-f21.tif
Fig. 21 Surface modified magnetic NPs for drug delivery. (A) Cisplatin can be encapsulated into the interior of hollow iron oxide NPs. The particle surface can be modified with PEGylated dopamine and then coupled with Herceptin for cancer cell targeting. (B) cytotoxicity studies. Compared with free cisplatin, cisplatin NPs led to much more efficient cancer cell killing. Figures are reprinted from ref. 143 with permission, Copyright 2009 American Chemical Society.

Heterodimer NPs have also been investigated as drug delivery vehicles. Unlike single component NPs, heterodimer NPs possess two surfaces, which is advantageous if multiple functionalities are to be imparted onto the particle surface. For instance, the Au–Fe3O4 NPs allow selective modification of Au and iron oxide surfaces by mercapto-PEG and dopamine-PEG, respectively.144 Cisplatin could be conjugated to the Au surface, along with a HER2 antibody tethered to iron oxide for cancer cell targeting. The resulting NPs showed increased toxicity compared to free cisplatin owing to selective delivery enabled by the NPs.145

The strong magnetism of iron oxide NPs may also permit magnet-guided drug delivery.146 For example, iron oxide NPs were loaded onto diatom shells and the resulting particles were investigated as a potential drug delivery vehicle. Diatoms are a major group of algae which are encased within a silica shell called a frustule. These diatom shells have a length of ∼10 μm with ∼500 nm pores on their surface. This unique feature allows them to encapsulate hundreds of magnetic NPs, and in doing so, endows the diatom with a superior magnetic response. In a proof-of-concept study, dye molecules as drug mimics were encapsulated along with iron oxide NPs into diatoms.146 These diatoms were i.v. injected into mice bearing subcutaneously inoculated tumours. Using fluorescence imaging and MRI, it was confirmed that enhanced tumour accumulation was achieved when an external magnetic field was applied to tumour areas.

Conclusions and future outlook

Advances in monodisperse NP synthesis and characterization have allowed nearly every application of NPs to flourish. In this review, we have discussed the syntheses of monodisperse NPs and their selected applications in catalysis and nanomedicine, both of which are of paramount importance to decipher any structure–property relationships of the NPs. We summarize recent advances of solution phase chemical syntheses of monodisperse NPs. Most of the syntheses follow the classical La Mer model on growing colloidal particles and require generally nucleation and growth stages for the formation of NPs to a desired size which should be further capped with surfactant(s) for NP stabilization in the reaction solution. The versatile solution phase chemistry allows fine-tuning of reaction parameters, leading to the formation of monodisperse NPs. Depending on synthetic conditions applied in the synthesis, the growth can yield thermodynamically stable polyhedral NPs, or kinetically controlled NPs with a designated shape. With controlled nucleation, the synthesis can be extended to grow shells on the seeding NPs (seed-mediated growth), making it possible to control not only NP sizes, but also heterostructured multicomponent systems for the formation of core/shell and dumbbell-like composite NPs.

This size, shape and complexity control realized in the synthesis yield NPs with more precise surface chemistry and physical properties which are important for the next step: applications. For example, NP catalysis can now be tuned and optimized for the oxygen reduction reaction, CO2 reduction and cascade dehydrogenation/hydrogenations to functional organic compounds under greener chemical synthesis conditions. NPs with tunable physical properties and controlled surface chemistry are also explored extensively for understanding NP chemistry in biological systems to achieve the desired NP biocompatibility, biocirculation, biodistribution, and bioelimination. In this review, we focus on highlighting monodisperse iron oxide NPs and their controlled surface functionalization for target-specific cancer imaging and anti-cancer drug delivery. These studies have demonstrated that monodisperse NPs have risen as the ideal model systems to determine how small changes on the nanoscale can affect NP properties and NP interactions with biology.

Despite the advances made in the synthesis and extensive studies devoted to monodisperse NPs, more challenges still exist and overcoming these challenges is essential for NPs to demonstrate practical uses. On the synthetic side, using solution phase synthesis is still difficult to produce monodisperse NPs at commercial scale. Those prepared and sold commercially tend to be less monodisperse than those demonstrated from lab scale synthesis. NPs do have intrinsic large surface energy, which often facilitates their binding nonselectively with any molecules present adjacent to them, making it very difficult to control/quantify NP surface chemistry. NPs with energetically unfavorable shapes or morphologies may not be stable, and as a result, the low-coordination atoms on the NP surface have high chemical potentials and tend to relax to find the low energy spots, degrading the shape, quality and properties of the NPs. Robust coatings are generally needed, which unfortunately often compromise the NP surface chemistry. In catalysis, it is extremely important to have a stable NP surface where a chemical reaction can be monitored and a catalytic pathway can be elucidated. However, the dynamic nature of the NP surface under the catalytic reaction conditions, especially under high temperature and corrosive conditions, makes it difficult to stabilize NPs for reaction observation/characterization. NP interactions with biomolecules are key for the NPs to be applicable to the proposed biomedicine uses to achieve sensitive biomedical imaging and efficient therapy, which relies essentially on developing NPs with predictable surface chemistry and biological interactions.

The encouraging news is that decades of efforts on NP studies have yielded methodologies which allow us to achieve unprecedented control on NP dimensions and properties. These pave the way for further studies on understanding NP stability, surface chemistry, surface reactivity, and bioconjugation. Monodisperse NPs will be utilized as successful model systems for understanding structure–property relationships and as practical catalysts or probes for advanced nanotechnological applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Work at Brown University was supported by the US Department of Energy under Contract No. DE-AC02-06CH11357, Fuel Cell Technologies Office, the American Chemical Society Petroleum Research Fund (57114-ND5), the Office of Vice President of Research of Brown University, the Institute of Molecular and Nanoscale Innovation of Brown University, and Strem Chemicals, as well as in part by the U.S. Army Research Laboratory and the U.S. Army Research Office under the Multi University Research Initiative MURI (W911NF-11-1-0353) on “Stress-Controlled Catalysis via Engineered Nanostructures,” the U.S. Army Research Laboratory and the U.S. Army Research Office under grant W911NF-15-1-0147, and the Center for the Capture and Conversion of CO2, a Center for Chemical Innovation funded by the National Science Foundation, CHE-1240020. M. M. is supported by the National Science Foundation Graduate Research Fellowship, under grant no. 1644760. Work at the University of Georgia was supported by the National Science Foundation (CAREER grant no. NSF1552617) and the National Institute of Biomedical Imaging and Bioengineering (grant no. R01EB022596).

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