Interaction of theory and experiment: examples from single molecule studies of nanoparticles

Nature of Science (NOS) Being Acquainted with Science of Science (SoS): Providing a Panoramic Picture of Sciences to Embody NOS for Pre-Service Teachers

Understanding about nature of science is important topic in science education as well as in pre-service science teacher education. In science education, Nature of Science (NOS), in its different forms of educational scaffoldings, seeks to provide with students an understanding of features of scientific knowledge and science in general, how scientific knowledge changes and becomes accepted, and what factors guide scientific activities. For a science teacher, deep and broad enough picture of sciences is therefore of importance. This study attempts to show that the research field called Science of Science (SoS) can significantly support building such a panoramic picture of sciences, and through that, significantly support NOS. The SoS approaches the structure and dynamics of science quantitatively, using scientific documents (e.g., publications, reports, books and monographs and patent applications) as trails to map the landscape of sciences. It is argued here that SoS may provide material and interesting cases for NOS, and in so doing enrich NOS in a similarly significant way as history, philosophy and sociology of science (HPSS) scholarship has done thus far. This study introduces several themes based on SoS that are of relevance for NOS as they were introduced and discussed in a pre-service science teachers’ course. The feedback from pre-service teachers shows that introducing SoS, with minimal additional philosophical interpretations and discussions, but simply as evidential facts and findings, sparks ideas and views that come very close to NOS themes and topics. Discussions related to nature of science, and specific educational NOS scaffoldings for it, can find a good companion in SoS; the latter providing facts and evidence of thee structure and dynamics of sciences, the former providing perspectives for interpretations.

Substance P in Solution: Trans-to-Cis Configurational Changes of Penultimate Prolines Initiate Non-enzymatic Peptide Bond Cleavages

We report ion mobility spectrometry and mass spectrometry studies of the non-enzymatic step-by-step degradation of substance P (subP), an 11-residue neuropeptide, with the sequence Arg¹-Pro²-Lys³-Pro⁴-Gln⁵-Gln⁶-Phe⁷-Phe⁸-Gly⁹-Leu¹⁰-Met¹¹-NH₂, in ethanol. At elevated solution temperatures (55 to 75 °C), several reactions are observed, including a protonation event, i.e., [subP+2H]²⁺ + H⁺ → [subP+3H]³⁺, that appears to be regulated by a configurational change and two sequential bond cleavages (the Pro²-Lys³ peptide bond is cleaved to form the smaller nonapeptide Lys³-Met¹¹-NH₂ [subP_(3-11)], and subsequently, subP_(3-11) is cleaved at the Pro⁴-Gln⁵ peptide bond to yield the heptapeptide Gln⁵-Met¹¹-NH₂ [subP_(5-11)]). Each of the product peptides [subP_(3-11) and subP_(5-11)] is accompanied by a complementary diketopiperazine (DKP): cyclo-Arg¹-Pro² (cRP) for the first cleavage, and cyclo-Lys³-Pro⁴ (cKP) for the second. Insight about the mechanism of degradation is obtained by comparing kinetics calculations of trial model mechanisms with experimental data. The best model of our experimental data indicates that the initial cleavage of subP is regulated by a conformational change, likely a trans→cis isomerization of the Arg¹-Pro² peptide bond. The subP_(3-11) product has a long lifetime (t_1/2 ~ 30 h at 55 °C) and appears to transition through several structural intermediates prior to dissociation, suggesting that subP_(3-11) is initially formed with a Lys³-trans-Pro⁴ peptide bond configuration and that slow trans→cis isomerization regulates the second bond cleavage event as well. From these data and our model mechanisms, we obtain transition state thermochemistry ranging from ΔH^‡ = 41 to 85 kJ mol^-1 and ΔS^‡ = - 43 to - 157 J mol^-1 K^-1 for each step in the reaction. Graphical Abstract.

Extension of the diffusion controlled electron transfer theory for intermittent fluorescence of quantum dots: inclusion of biexcitons and the difference of "on" and "off" time distributions

The equations for the diffusion controlled electron transfer (DCET) theory of quantum dot blinking are extended to include biexcitons. In contrast to excitons, which undego resonant light to dark transitions, the biexcitons, having a much larger total energy, undergo a Fermi's Golden rule type transfer (many acceptance states). The latter immediately gives rise to an exponential tail for the light state, and it is explained why the dark state power law behavior is unaffected. Results are given for both continuous and pulsed excitation. The typical -3/2 power law for the light state at low light intensities, and for the dark state at all intensities, as well as dependence of the exponential tail on the square of the light intensity, and a decrease of the power in the power law for the light state from -3/2 to less negative values with increasing light intensity are all consistent with the theory. The desirability of measuring the dependence of the spectral diffusion coefficient on light intensity at room temperature as a test of several aspects of the theory is noted.

Unique temporal and spatial biomolecular emission profile on individual zinc oxide nanorods

Zinc oxide nanorods (ZnO NRs) have emerged in recent years as extremely useful, optical signal-enhancing platforms in DNA and protein detection. Although the use of ZnO NRs in biodetection has been demonstrated so far in systems involving many ZnO NRs per detection element, their future applications will likely take place in a miniaturized setting while exploiting single ZnO NRs in a low-volume, high-throughput bioanalysis. In this paper, we investigate temporal and spatial characteristics of the biomolecular fluorescence on individual ZnO NR systems. Quantitative and qualitative examinations of the biomolecular intensity and photostability are carried out as a function of two important criteria, the time and position along the long axis (length) of NRs. Photostability profiles are also measured with respect to the position on NRs and compared to those characteristics of biomolecules on polymeric control platforms. Unlike the uniformly distributed signal observed on the control platforms, both the fluorescence intensity and photostability are position-dependent on individual ZnO NRs. We have identified a unique phenomenon of highly localized, fluorescence intensification on the nanorod ends (FINE) of well-characterized, individual ZnO nanostructures. When compared to the polymeric controls, the biomolecular fluorescence intensity and photostability are determined to be higher on individual ZnO NRs regardless of the position on NRs. We have also carried out finite-difference time-domain simulations the results of which are in good agreement with the observed FINE. The outcomes of our investigation will offer a much needed basis for signal interpretation for biodetection devices and platforms consisting of single ZnO NRs and, at the same time, contribute significantly to provide insight in understanding the biomolecular fluorescence observed from ZnO NR ensemble-based systems.

The possibility and implications of dynamic nanoparticle surfaces

Understanding the precise nature of a surface or interface is a key component toward optimizing the desired properties and function of a material. For semiconductor nanocrystals, the surface has been shown to modulate fluorescence efficiency, lifetime, and intermittency. The theoretical picture of a nanocrystal surface has included the existence of an undefined mixture of trap states that arise from incomplete passivation. However, our recent scanning transmission electron microscope movies and supporting theoretical evidence suggest that, under excitation, the surface is fluctuating, creating a dynamic population of surface and subsurface states. This possibility challenges our fundamental understanding of the surface and could have far-reaching ramifications for nanoparticle-based technologies. In this Perspective, we discuss the current theories behind the optical properties of nanocrystals in the context of fluxionality.