Summary Individual mammalian cells exhibit large variability in cellular volume even with the same absolute DNA content and so must compensate for differences in DNA concentration in order to maintain constant concentration of gene expression products. Using single molecule counting and computational image analysis, we show that transcript abundance correlates with cellular volume at the single cell level due to increased global transcription in larger cells. Cell fusion experiments establish that increased cellular content itself can directly increase transcription. Quantitative analysis shows that this mechanism measures the ratio of cellular volume to DNA content, mostly likely through sequestration of a transcriptional factor to DNA. Analysis of transcriptional bursts reveals a separate mechanism for gene dosage compensation after DNA replication that enables proper transcriptional output during early and late S-phase. Our results provide a framework for quantitatively understanding the relationships between DNA content, cell size and gene expression variability in single cells.

We report here an assessment of carrier multiplication (CM) yields in PbSe and PbS nanocrystals (NCs) by a quantitative analysis of biexciton and exciton dynamics in transient photoluminescence decays. Interest in CM, the generation of more than one electron and hole in a semiconductor after absorption of one photon, has renewed in recent years because of reports suggesting greatly increased efficiencies in nanocrystalline materials compared to the bulk form, in which CM was otherwise too weak to be of consequence in photovoltaic energy conversion devices. In our PbSe and PbS NC samples, however, we estimate using transient photoluminescence that at most 0.25 additional eh pairs are generated per photon even at energies ω > 5Eg, instead of the much higher values reported in the literature. We argue by comparing NC CM estimates and reported bulk values on an absolute energy basis, which we justify as appropriate on physical grounds, that the data reported thus far are inconclusive with respect to the importance of nanoscale-specific phenomena in the CM process. arXiv: 0806.1966v1 [cond-mat.mtrl-sci]

The development of a reversible chemical sensor based on a CdSe/ZnS nanocrystal (NC) is described. Signal transduction is accomplished by fluorescence resonance energy transfer (FRET) between the NC and a fluorescent pH-sensitive squaraine dye attached to the surface of the NC. The efficiency of FRET, and consequently the relative intensity of NC and dye emissions, is modulated with the pH-dependent absorption cross section of the squaraine dye. The design of a NC sensor based on FRET results in a ratiometric sensor since the emission intensities of dye and NC may be referenced to the isosbestic point between NC and dye emissions. The ratiometric approach allows sensing to be performed, regardless of issues surrounding collection efficiency (scattering environment, light fluctuations, etc.) and dye:NC loadings.

Biexciton properties strongly affect the usability of a light emitter in quantum photon sources and lasers but are difficult to measure for single fluorophores at room temperature due to luminescence intermittency and bleaching at the high excitation fluences usually required. Here, we observe the biexciton (BX) to exciton (X) to ground photoluminescence cascade of single colloidal semiconductor nanocrystals (NCs) under weak excitation in a g(2) photon correlation measurement and show that the normalized amplitude of the cascade feature is equal to the ratio of the BX to X fluorescence quantum yields. This imposes a limit on the attainable depth of photon antibunching and provides a robust means to study single emitter biexciton physics. In NC samples, we show that the BX quantum yield is considerably inhomogeneous, consistent with the defect sensitivity expected of the Auger nonradiative recombination mechanism. The method can be extended to study X,BX spectral and polarization correlations.

Semiconductor nanocrystals emit light intermittently; i.e., they “blink,” under steady illumination. The dark periods have been widely assumed to be due to photoluminescence (PL) quenching by an Auger-like process involving a single additional charge present in the nanocrystal. Our results challenge this long-standing assumption. Close examination of exciton PL intensity time traces of single CdSe(CdZnS) core (shell) nanocrystals reveals that the dark state PL quantum yield can be 10 times less than the biexciton PL quantum yield. In addition, we observe spectrally resolved multiexciton emission and find that it also blinks with an on/off ratio greater than 10:1. These results directly contradict the predictions of the charging model.