Rapid Nanoparticle Synthesis by Magnetic and Microwave Heating

Citation: Chikan, V., & McLaurin, E. J. (2016). Rapid Nanoparticle Synthesis by Magnetic and Microwave Heating. Nanomaterials, 6(5), 9. doi:10.3390/nano6050085Traditional hot-injection (HI) syntheses of colloidal nanoparticles (NPs) allows good separation of the nucleation and growth stages of the reaction, a key limitation in obtaining monodisperse NPs, but with limited scalability. Here, two methods are presented for obtaining NPs via rapid heating: magnetic and microwave-assisted. Both of these techniques provide improved engineering control over the separation of nucleation and growth stages of nanomaterial synthesis when the reaction is initiated from room temperature. The advantages of these techniques with preliminary data are presented in this prospective article. It is shown here that microwave assisted heating could possibly provide some selectivity in activating the nanomaterial precursor materials, while magnetic heating can produce very tiny particles in a very short time (even on the millisecond timescale), which is important for scalability. The fast magnetic heating also allows for synthesizing larger particles with improved size distribution, therefore impacting, not only the quantity, but the quality of the nanomaterials

Phosphorescent semiconductor nanocrystals and proteins for biological oxygen sensing

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2011.Vita. Cataloged from PDF version of thesis.Includes bibliographical references.Oxygen is required for cellular respiration by all complex life making it a key metabolic profiling factor in biological systems. Tumors are defined by hypoxia (low pO2), which has been shown to influence response to radiation therapy and chemotheraphy. However, very little is known about spatio-temporal changes in P0 2 during tumor progression and therapy. To fully characterize and probe the tumor microenvironment, new tools are needed to quantitatively assess the microanatonical and physiological changes occurring during tumor growth and treatment. This thesis explores the design and construction of new oxygen sensors as tools for monitoring the tumor microenvironment in real-time. Semiconductor nanocrystals or quantum dots (QDs) are the basis of these tools. Previously, most imaging applications of QDs have used them as indicators of position; they have lacked a response to their local environment. Tethering a phosphorescent complex to a QD enables fluorescence resonance energy transfer to be exploited as a signal transduction mechanism, sensitizing the QD to oxygen. The mechanism for oxygen sensing involves kinetic quenching of the emission of the energy accepting phosphor in the presence of oxygen, while the emission of the energy donating QD remains stable. This mechanism was chosen owing to the unique ability of oxygen to quench emission from a phosphorescent compound, but not fluorescence from a QD. Phosphors such as osmium polypyridines (Chapter 2), Pd or Pt porphyrins (Chapters 3 and 4), or phosphorescent proteins (Chapters 5 and 6) may all be employed. An additional benefit of FRET excitation includes very large one- and two-photon excitation cross-sections of QDs. Together, these properties make the probes ideal candidates for 02 sensing applications in biological microenvironments, where probe concentrations may vary, and where the use of multiphoton excitation in microscopy presents significant advantages in imaging thick samples and in limiting extraneous tissue damage.by Emily J. McLaurin.Ph.D

Rapid Induction and Microwave Heat-Up Syntheses of CdSe Quantum Dots

The production of nanoparticles on an industrial scale requires an approach other than the widely used hot-injection method. In this work, two heat-up methods are applied to nanoparticle synthesis. The induction heating method produces CdSe quantum dots with ultrasmall properties in seconds. Initial flow-through experiments demonstrate that induction heating continuously produces quantum dots. These results are compared with those from microwave synthesis, which produces quantum dots on a longer timescale but provides fast, continuous heating. Both methods can produce quantum dots within seconds because of rapid heating. In addition, different precursors, single source and separate source, give different results, ultimately providing a handle to control quantum dot properties. © Copyright 2018 American Chemical Society

Ru-Porphyrin Protein Scaffolds for Sensing O2

Hemoprotein-based scaffolds containing phosphorescent ruthenium(II) CO mesoporphyrin IX (RuMP) are reported here for oxygen (O2) sensing in biological contexts. RuMP was incorporated into the protein scaffolds during protein expression utilizing a novel method that we have described previously. A high-resolution (2.00 Å) crystal structure revealed that the unnatural porphyrin binds to the proteins in a manner similar to the native heme and does not perturb the protein fold. The protein scaffolds were found to provide unique coordination environments for RuMP and modulate the porphyrin emission properties. Emission lifetime measurements demonstrate a linear O2 response within the physiological range and precision comparable to commercial O2 sensors. The RuMP proteins are robust, readily-modifiable platforms and display promising O2 sensing properties for future in vivo applications.Chemistry and Chemical Biolog

InP Nanocrystals with Color-Tunable Luminescence by Microwave-Assisted Ionic-Liquid Etching

This article reports the successful synthesis of luminescent InP nanocrystals (NCs) of different sizes using microwave-assisted methods. The key to NC photoluminescence (PL) is the use of 1-butyl-4-methylpyridinium tetrafluoroborate (BMPy BF₄), an ionic liquid (IL) with a fluoride-containing anion. The IL is responsible for both rapid heating of the reaction solution and efficient PL from the NCs. This is demonstrated by comparison with other ionic liquids, an analogous synthesis done using a flask and heating mantle, and a microwave-assisted synthesis in a silicon carbide (SiC) vessel. Addition of amine results in InP NC sizes ranging from ∼3.2–4.2 nm, calculated (PL from ∼545–630 nm) with quantum yields (QYs) of up to 30% and PL full-width-at-half-max (fwhm) values as small as ∼0.18 eV (∼59 at 632 nm) without optimization. By taking advantage of efficient microwave-assisted heating, this simple procedure provides a pathway to luminescent InP NCs of different diameters without changing precursors, performing sequential additions of reactants, or other postreaction processing (e.g., shell growth, HF etching)

Luminescence Saturation via Mn 2+ –Exciton Cross Relaxation in Colloidal Doped Semiconductor Nanocrystals

Colloidal Mn2+-doped semiconductor nanocrystals such as Mn2+:ZnSe have attracted broad attention for potential applications in phosphor and imaging technologies. Here, we report saturation of the sensitized Mn2+ photoluminescence intensity at very low continuous-wave (CW) and quasi-CW photoexcitation powers under conditions that are relevant to many of the proposed applications. Time-resolved photoluminescence measurements and kinetic modeling indicate that this saturation arises from an Auger-type nonradiative cross relaxation between an excited Mn2+ ion and an exciton within the same nanocrystal. A lower limit of k = 2 × 1010 s–1 is established for the fundamental rate constant of the Mn2+(4T1)-exciton cross relaxation

Two-Photon Absorbing Nanocrystal Sensors for Ratiometric Detection of Oxygen

Two nanocrystal-osmium(II) polypyridyl (NC-Os(II)PP) conjugates have been designed to detect oxygen in biological environments. Polypyridines appended with a single free amine were linked with facility to a carboxylic acid functionality of a semiconductor NC overlayer to afford a biologically stable amide bond. The Os(II)PP complexes possess broad absorptions that extend into the red spectral region; this absorption feature makes them desirable acceptors of energy from NC donors. Fluorescence resonance energy transfer (FRET) from the NC to the Os(II)PP causes an enhanced Os(II)PP emission with a concomitant quenching of the NC emission. Owing to the large two-photon absorption cross-section of the NCs, FRET from NC to the Os(II)PP can be established under two-photon excitation conditions. In this way, two-photon processes of metal polypyridyl complexes can be exploited for sensing. The emission of the NC is insensitive to oxygen, even at 1 atm, whereas excited states of both osmium complexes are quenched in the presence of oxygen. The NC emission may thus be used as an internal reference to correct for fluctuations in the photoluminescence intensity signal. These properties taken together establish NC-Os(II)PP conjugates as competent ratiometric, two-photon oxygen sensors for application in biological microenvironments.Chemistry and Chemical Biolog

Mn2+-ZnSe/ZnS@SiO2 Nanoparticles for Turn-on Luminescence Thiol Detection

Biological thiols are antioxidants essential for the prevention of disease. For example, low levels of the tripeptide glutathione are associated with heart disease, cancer, and dementia. Mn2+-doped wide bandgap semiconductor nanocrystals exhibit luminescence and magnetic properties that make them attractive for bimodal imaging. We found that these nanocrystals and silica-encapsulated nanoparticle derivatives exhibit enhanced luminescence in the presence of thiols in both organic solvent and aqueous solution. The key to using these nanocrystals as sensors is control over their surfaces. The addition of a ZnS barrier layer or shell produces more stable nanocrystals that are isolated from their surroundings, and luminescence enhancement is only observed with thinner, intermediate shells. Tunability is demonstrated with dodecanethiol and sensitivities decrease with thin, medium, and thick shells. Turn-on nanoprobe luminescence is also generated by several biological thiols, including glutathione, N-acetylcysteine, cysteine, and dithiothreitol. Nanoparticles prepared with different ZnS shell thicknesses demonstrated varying sensitivity to glutathione, which allows for the tuning of particle sensitivity without optimization. The small photoluminescence response to control amino acids and salts indicates selectivity for thiols. Preliminary magnetic measurements highlight the challenge of optimizing sensors for different imaging modalities. In this work, we assess the prospects of using these nanoparticles as luminescent turn-on thiol sensors and for MRI

Two-Photon Absorbing Nanocrystal Sensors for Ratiometric Detection of Oxygen

Two nanocrystal-osmium(II) polypyridyl (NC-Os(II)PP) conjugates have been designed to detect oxygen in biological environments. Polypyridines appended with a single free amine were linked with facility to a carboxylic acid functionality of a semiconductor NC overlayer to afford a biologically stable amide bond. The Os(II)PP complexes possess broad absorptions that extend into the red spectral region; this absorption feature makes them desirable acceptors of energy from NC donors. Fluorescence resonance energy transfer (FRET) from the NC to the Os(II)PP causes an enhanced Os(II)PP emission with a concomitant quenching of the NC emission. Owing to the large two-photon absorption cross-section of the NCs, FRET from NC to the Os(II)PP can be established under two-photon excitation conditions. In this way, two-photon processes of metal polypyridyl complexes can be exploited for sensing. The emission of the NC is insensitive to oxygen, even at 1 atm, whereas excited states of both osmium complexes are quenched in the presence of oxygen. The NC emission may thus be used as an internal reference to correct for fluctuations in the photoluminescence intensity signal. These properties taken together establish NC-Os(II)PP conjugates as competent ratiometric, two-photon oxygen sensors for application in biological microenvironments

Rapid Induction and Microwave Heat-Up Syntheses of CdSe Quantum Dots

The production of nanoparticles on an industrial scale requires an approach other than the widely used hot-injection method. In this work, two heat-up methods are applied to nanoparticle synthesis. The induction heating method produces CdSe quantum dots with ultrasmall properties in seconds. Initial flow-through experiments demonstrate that induction heating continuously produces quantum dots. These results are compared with those from microwave synthesis, which produces quantum dots on a longer timescale but provides fast, continuous heating. Both methods can produce quantum dots within seconds because of rapid heating. In addition, different precursors, single source and separate source, give different results, ultimately providing a handle to control quantum dot properties