Erin A. Riley

Beyond power laws: A new approach for analyzing single molecule photoluminescence intermittency

The photoluminescence intermittency (PI) exhibited by single emitters has been studied for over a decade. To date, the vast majority of PI analyses involve parsing the data into emissive and non-emissive events, constructing histograms of event durations, and fitting these histograms to either exponential or power law probability distributions functions (PDFs). Here, a new method for analyzing PI data is presented where the data are used directly to construct a cumulative distribution function (CDF), and maximum-likelihood estimation techniques are used to determine the best fit of a model PDF to the CDF. Statistical tests are then employed to quantitatively evaluate the hypothesis that the CDF (data) is represented by the model PDF. The analysis method is outlined and applied to PI exhibited by single CdSe∕CdS core-shell nanocrystals and the organic chromophore violamine R isolated in single crystals of potassium-acid phthalate. Contrary to previous studies, the analysis presented here demonstrates that the PI exhibited by these systems is not described by a power law. The analysis developed here is also used to quantify heterogeneity within PI data obtained from a collection of CdSe/CdS nanocrytals, and for the determination of statistically significant changes in PI accompanying perturbation of the emitter. In summary, the analysis methodology presented here provides a more statistically robust approach for analyzing PI data.

Photoluminescence Intermittency from Single Quantum Dots to Organic Molecules: Emerging Themes

Recent experimental and theoretical studies of photoluminescence intermittency (PI) or “blinking” exhibited by single core/shell quantum dots and single organic luminophores are reviewed. For quantum dots, a discussion of early models describing the origin of PI in these materials and recent challenges to these models are presented. For organic luminophores the role of electron transfer, proton transfer and other photophysical processes in PI are discussed. Finally, new experimental and data analysis methods are outlined that promise to be instrumental in future discoveries regarding the origin(s) of PI exhibited by single emitters.

Urinary Polycyclic Aromatic Hydrocarbon Metabolites as Biomarkers to Woodsmoke Exposure – Results from a Controlled Exposure Study

Woodsmoke contains harmful components — such as fine particulate matter (PM2.5) and polycyclic aromatic hydrocarbons (PAHs) — and impacts more than half of the global population. We investigated urinary hydroxylated PAH metabolites (OH-PAHs) as woodsmoke exposure biomarkers in nine non-smoking volunteers experimentally exposed to a wood fire. Individual urine samples were collected from 24-h before to 48-h after the exposure and personal PM2.5 samples were collected during the 2-h woodsmoke exposure. Concentrations of nine OH-PAHs increased by 1.8–7.2 times within 2.3–19.3 h, and returned to baseline approximately 24 h after the exposure. 2-Naphthol (2-NAP) had the largest post-exposure increase and exhibited a clear excretion pattern in all participants. The level of urinary OH-PAHs, except 1-hydroxypyrene (1-PYR), correlated with those of PM2.5, levoglucosan and PAHs in personal PM2.5 samples. This finding suggests that several urinary OH-PAHs, especially 2-NAP, are potential exposure biomarkers to woodsmoke; by contrast, 1-PYR may not be a suitable biomarker. Compared with levoglucosan and methoxyphenols — two other urinary woodsmoke biomarkers that were measured in the same study and reported previously — OH-PAHs might be better biomarkers based on sensitivity, robustness and stability, particularly under suboptimal sampling and storage conditions, like in epidemiological studies carried out in less developed areas.

Proton transfer and photoluminescence intermittency of single emitters in dyed crystals.

The role of proton transfer in the photoluminescence intermittency (PI) of single molecules of violamine R (VR) overgrown in potassium acid phthalate (KAP) crystals is evaluated in comparisons of protonated (KAP) and deuterated (DKAP) mixed crystals between 23 and 60 °C. The PI is analyzed by the construction of cumulative distribution functions that are statistically compared. We find that the on- and off-interval duration distributions change with isotopic substitution consistent with proton transfer contributing to the PI of VR. The on- and off-interval duration distributions have distinct temperature dependencies consistent with different mechanisms for dark state production and decay. Additional evidence for proton-transfer is provided by distributions of single molecule emission-energy maxima that reflect emission from protonated and deprotonated VR. A mechanism for the PI of KAP is presented, where the dark state is assigned to formation of the colorless, leuco form of VR, formed by proton transfer from VR to the KAP lattice, and decay of the dark state involves ring-opening promoted by proton transfer from KAP to VR. The distributed kinetics for dark-state production and decay are modeled using a log-normal distribution for the PI data in preference to a power-law previously assumed. A discussion of the log-normal distribution with regards to PI and proton transfer is presented.

Dielectric dependence of single-molecule photoluminescence intermittency: nile red in poly(vinylidene fluoride).

The dependence of single-molecule photoluminescence intermittency (PI) or “blinking” on the local dielectric constant (ε) is examined for nile red (NR) in thin films of poly(vinylidene fluoride) (PVDF). In previous studies, variation of the local dielectric constant was accomplished by studying luminophores in chemically and structurally different hosts. In contrast, the NR/PVDF guest–host pair allows for the investigation of PI as a function of ε while keeping the chemical composition of both the luminophore and host unchanged. The solvatochromic properties of NR are used to measure the local ε, while fluctuations in NR emission intensity over time provide a measure of the PI. PVDF is an ideal host for this study because it provides submicron-sized dielectric domains that vary from nonpolar (ε ≈ 2) to very polar (ε ≈ 70). The results presented here demonstrate that the local dielectric environment can have a pronounced effect on PI. We find that the NR emissive events increase 5-fold with an increase in ε from 2.2 to 74. A complex dependence on ε is also observed for NR nonemissive event durations, initially increasing as ε increases from 2.2 to 3.4 but decreasing in duration with further increase in ε. The variation in emissive event durations with ε is reproduced using a photoinduced electron-transfer model involving electron transfer from NR to PVDF. In addition, an increase in NR photostability with an increase in ε is observed, suggesting that the dielectric environment plays an important role in defining the photostability of NR in PVDF.

Temperature-dependent fluorescence intermittency for single molecules of violamine R in poly(vinyl alcohol)

The fluorescence intermittency or blinking of violamine R (VR) in poly(vinyl alcohol) (PVA) is measured for temperatures spanning the glass-transition temperature (Tg) of the polymer host. Probability distributions of on- and off-time durations are acquired and analyzed for temperatures ranging from 22.5 °C (Tg − 49 °C) to 85 °C (Tg + 13 °C). At all temperatures, the probability distributions are well described by a power-law. The on-time power-law exponent (mon) undergoes a modest increase with temperature, and the off-time power-law exponent (moff) varies little with temperature. Both observations are consistent with current electron transfer models for the production and decay of the non-emissive state. Surprisingly, both mon and moff demonstrated smooth variation as the temperature transitions through Tg. This observation suggests that the local VR environment is not significantly altered over the temperature range investigated here. The onset of deviation from power-law behavior is observed at Tg + 13 °C, consistent with thermally enhanced environmental fluctuations providing for the interchange of local environments that is rapid compared to blinking.

Evaluation of 1-Nitropyrene as a Surrogate Measure for Diesel Exhaust

We investigated the viability of particle bound 1-nitropyrene (1-NP) air concentration measurements as a surrogate of diesel exhaust (DE) exposure, as compared with industry-standard elemental carbon (EC) and total carbon (TC) measurements. Personal exposures are reported for 18 employees at a large underground metal mine during four different monitoring campaigns. Full-shift personal air exposure sampling was conducted using a Mine Safety and Health Administration (MSHA) compliant diesel particulate matter (DPM) impactor cassette downstream of a GS-1 cyclone pre-selector. Each DPM filter element was analyzed for EC and organic carbon (OC) using NIOSH Method 5040. After EC and OC analysis, the remaining portion of each DPM filter was analyzed for 1-NP using liquid chromatography tandem mass spectrometry (LC/MS/MS). We observed high correlations between the quantiles of 1-NP and EC exposures across 10 different work shift task groups (r = 0.87 to 0.96), and a linear relationship with a slope between 6.0 to 6.9 pg 1-NP per µg EC. However, correlation between 1-NP and EC was weak (r =0.34) for the 91 individual sample pairs due to low EC concentrations and possible heterogeneity of DE composition. While both 1-NP and EC differentiated between high and low exposure groups categorized by job location, measurements of 1-NP, but not EC further differentiated between specific job activities. Repeated measurements on individual subjects verified the relationship between 1-NP and EC and demonstrated substantial within-subject variability in exposure. The detection limit of TC air concentration ranged between 18 and 28 µg m-3 and was limited by OC contamination of the quartz filters in the MSHA compliant DPM samplers.

Use of mobile and passive badge air monitoring data for NOX and ozone air pollution spatial exposure prediction models.

Air pollution exposure prediction models can make use of many types of air monitoring data. Fixed location passive samples typically measure concentrations averaged over several days to weeks. Mobile monitoring data can generate near continuous concentration measurements. It is not known whether mobile monitoring data are suitable for generating well-performing exposure prediction models or how they compare with other types of monitoring data in generating exposure models. Measurements from fixed site passive samplers and mobile monitoring platform were made over a 2-week period in Baltimore in the summer and winter months in 2012. Performance of exposure prediction models for long-term nitrogen oxides (NOX) and ozone (O3) concentrations were compared using a state-of-the-art approach for model development based on land use regression (LUR) and geostatistical smoothing. Model performance was evaluated using leave-one-out cross-validation (LOOCV). Models performed well using the mobile peak traffic monitoring data for both NOX and O3, with LOOCV R2s of 0.70 and 0.71, respectively, in the summer, and 0.90 and 0.58, respectively, in the winter. Models using 2-week passive samples for NOX had LOOCV R2s of 0.60 and 0.65 in the summer and winter months, respectively. The passive badge sampling data were not adequate for developing models for O3. Mobile air monitoring data can be used to successfully build well-performing LUR exposure prediction models for NOX and O3 and are a better source of data for these models than 2-week passive badge data.

Single-molecule studies of proton transfer in guest-host materials

Although the promise of organic materials for a variety of photonic applications has been recognized for some time, the wide-spread use of these materials is limited by photochemical processes that result in irreversible material decomposition. In this talk, the use of single-molecule microscopy to investigate the photochemistry of organic molecules in guest-host systems is described. A new method for analyzing the photoluminescence intermittency (PI) or “blinking” exhibited by single emitters is presented. This method allows for a statistically robust method for analyzing PI data, and for determining if an external perturbation results in a significant modification of the blinking statistcs. This method is applied to the blinking exhibited by CdSe/CdS quantum dots (QDs) in poly(methyl methacrylate) and the rhodamine derivative violamine R (VR) isolated in single crystals of potassium acid phthalate (KAP). For the QDs our analysis demonstrates that the blinking statistics are not power-law distributed. For VR in KAP we find that deuterium substitution significantly alters the PI exhibited by VR consistent with proton-transfer contributing to PI.