Fluorescence Correlation Spectroscopy Studies Research Articles

Unveiling the Influence of Brightness Heterogeneity in Fluorescence Correlation Spectroscopy of Perovskite Nanocrystals.

Nanoparticles (NPs), including perovskite nanocrystals (PNCs) with single photon purity, present challenges in fluorescence correlation spectroscopy (FCS) studies due to their distinct photoluminescence (PL) behaviors. In particular, the zero-time correlation amplitude [g2(0)] and the associated diffusion timescale (τD) of their FCS curves show substantial dependency on pump intensity (IP). Optical saturation inadequately explains the origin of this FCS phenomenon in NPs, thus setting them apart from conventional dye molecules, which do not manifest such behavior. This observation is apparently attributed to either photo-brightening or optical trapping, both lead to increased NP occupancy (N) in the excitation volume, consequently reducing the g2(0) amplitude [since g2(0) α 1/N] at high IP. However, an advanced FCS study utilizing alternating laser excitation at two different intensities dismisses such possibilities. Further investigation into single-particle blinking behaviors as a function of IP reveals that the intensity dependence of g2(0) primarily arises from the brightness heterogeneity prevalent in almost all types of NPs. This report delves into the complexities of the photophysical properties of NPs and their adverse impacts on FCS studies.

Deeper Insights into Mixed Crowding through Enzyme Activity, Dynamics, and Crowder Diffusion.

Given the fact that the cellular interior is crowded by many different kinds of macromolecules, it is important that in vitro studies be carried out in the presence of mixed crowder systems. In this regard, we have used binary crowders formed by the combination of some of the commonly used crowding agents, namely, Ficoll 70, Dextran 70, Dextran 40, and PEG 8000 (PEG 8), to study how these affect enzyme activity, dynamics, and crowder diffusion. The enzyme chosen is AK3L1, an isoform of adenylate kinase. To investigate its dynamics, we have carried out three single point mutations (A74C, A132C, and A209C) with the cysteine residues being labeled with a coumarin-based solvatochromic probe [CPM: (7-diethylamino-3-(4-maleimido-phenyl)-4-methylcoumarin)]. Both enzyme activity and dynamics decreased in the binary mixtures as compared with the sum of the individual crowders, suggesting a reduction in excluded volume (in the mixture). To gain deeper insights into the binary mixtures, fluorescence correlation spectroscopy studies were carried out using fluorescein isothiocyanate-labeled Dextran 70 and tetramethylrhodamine-labeled AK3L1 as the diffusion probes. Diffusion in binary mixtures was observed to be much more constrained (relative to the sum of the individual crowders) for the labeled enzyme as compared to the labeled crowder showing different environments being faced by the two species. This was further confirmed during imaging of the phase-separated droplets formed in the binary mixtures having PEG as one of the crowding agents. The interior of these droplets was found to be rich in crowders and densely packed, as shown by confocal and digital holographic microscopy images, with the enzymes predominantly residing outside these droplets, that is, in the relatively less crowded regions. Taken together, our data provide important insights into various aspects of the simplest form of mixed crowding, that is, composed of just two components, and also hint at the enhanced complexity that the cellular interior presents toward having a detailed and comprehensive understanding of the same.

Facet Dependent Photoluminescence Blinking from Perovskite Nanocrystals.

Photoluminescence (PL) blinking of nanoparticles, while detrimental to their imaging applications, may benefit next-generation displays if the blinking is precisely controlled by reversible electron/hole injections from an external source. Considerable efforts are made to create well-characterized charged excitons within nanoparticles through electrochemical charging, which has led to enhanced control over PL-blinking in numerous instances. Manipulating the photocharging/discharging rates in nanoparticles by surface engineering can represent a straightforward method for regulating their blinking behaviors, an area largely unexplored for perovskite nanocrystals (PNCs). This work shows facet engineering leading to different morphologies of PNCs characterized by distinct blinking patterns. For instance, examining the PL intensity trajectories of single PNCs, representing the instantaneous photon count rate over time, reveals that the OFF-state population significantly increases as the number of facets increases from six to twenty-six. This study suggests that extra-faceted PNCs, owing to their polar facets and expanded surface area, render them more susceptible to photocharging, which results in larger OFF-state populations. Furthermore, the fluorescence correlation spectroscopy (FCS) study unveils that the augmented propensity for photocharging in extra-faceted PNCs can also originate from their greater tendency to form complexes with neighboring molecules.

YidC from Escherichia coli Forms an Ion-Conducting Pore upon Activation by Ribosomes.

The universally conserved protein YidC aids in the insertion and folding of transmembrane polypeptides. Supposedly, a charged arginine faces its hydrophobic lipid core, facilitating polypeptide sliding along YidC's surface. How the membrane barrier to other molecules may be maintained is unclear. Here, we show that the purified and reconstituted E. coli YidC forms an ion-conducting transmembrane pore upon ribosome or ribosome-nascent chain complex (RNC) binding. In contrast to monomeric YidC structures, an AlphaFold parallel YidC dimer model harbors a pore. Experimental evidence for a dimeric assembly comes from our BN-PAGE analysis of native vesicles, fluorescence correlation spectroscopy studies, single-molecule fluorescence photobleaching observations, and crosslinking experiments. In the dimeric model, the conserved arginine and other residues interacting with nascent chains point into the putative pore. This result suggests the possibility of a YidC-assisted insertion mode alternative to the insertase mechanism.

Overcoming Barriers Associated with Oral Delivery of Differently Sized Fluorescent Core-Shell Silica Nanoparticles.

Oral delivery, while a highly desirable form of nanoparticle-drug administration, is limited by challenges associated with overcoming several biological barriers. Here, the authors study how fluorescent and poly(ethylene glycol)-coated (PEGylated) core-shell silica nanoparticles sized 5 to 50nm interact with major barriers including intestinal mucus, intestinal epithelium, and stomach acid. From imaging fluorescence correlation spectroscopy studies using quasi-total internal reflection fluorescence microscopy, diffusion of nanoparticles through highly scattering mucus is progressively hindered above a critical hydrodynamic size around 20nm. By studying Caco-2 cell monolayers mimicking the intestinal epithelia, it is observed that ultrasmall nanoparticles below 10nm diameter (Cornell prime dots, [C' dots]) show permeabilities correlated with high absorption in humans from primarily enhanced passive passage through tight junctions. Particles above 20nm diameter exclusively show active transport through cells. After establishing C' dot stability in artificial gastric juice, in vivo oral gavage experiments in mice demonstrate successful passage through the body followed by renal clearance without protein corona formation. Results suggest C' dots as viable candidates for oral administration to patients with a proven pathway towards clinical translation and may generate renewed interest in examining silica as a food additive and its effects on nutrition and health.

Fluorescence Correlation Spectroscopy Studies of Dye Diffusion on Fresh and Aged Polyethylene Terephthalate.

Microplastics accumulate a wide variety of organic pollutants and thus may serve as efficient vectors for the transport of toxic substances. Much remains to be learned about how organic molecules interact with the surfaces of plastics and how these properties evolve as the microplastics are weathered. In this Letter, we report, for the first time, the application of confocal fluorescence correlation spectroscopy (FCS) to studies of organic molecules adsorbed from aqueous solution onto the surfaces of synthetic secondary microplastics. Both fresh and artificially aged poly(ethylene terephthalate) (PET) plastics are employed. The plastics are artificially aged in a UV-ozone chamber. Raman and infrared spectra confirm the composition of the PET microplastics. Water contact angle and surface roughness measurements reveal, respectively, an increase in wettability and a change in the nature of roughness with aging, consistent with surface oxidation. Rhodamine B (RhB) dye is used as a fluorescent probe in FCS studies and serves as an analogue for organic pollutants commonly found on microplastics. The FCS results reveal the accumulation of dye on the PET surfaces as they age. Dye motion is significantly slower on the plastics than in bulk aqueous solution and occurs by anomalous subdiffusion. The rate of diffusion becomes dramatically slower and more anomalous as the plastics are aged. Surface diffusion is likely slowed by either ionic interactions or hydrogen bonding between the dye and plastic. These results provide new insights critical to the understanding of how microplastics accumulate and transport organic pollutants as they weather in the environment.

Dodecahedron CsPbBr3 Perovskite Nanocrystals Enable Facile Harvesting of Hot Electrons and Holes

This Letter reports the facile harvesting of hot carriers (HCs) in a composite of 12-faceted dodecahedron CsPbBr3 nanocrystal (NC) and a scavenger molecule. We recorded ∼3.3 × 1011 s-1 HC cooling rate in NC when excited with ∼1.4 times the band gap energy (Eg), increasing to >3 × 1012 s-1 in the presence of scavengers at high concentration due to the HC extractions. Since the observed intrinsic charge transfer rate (∼1.7 × 1012 s-1) in our NC-scavenger complex is about an order of magnitude higher than the HC cooling rate (∼3.3 × 1011 s-1), carriers are harvested before their cooling. Further, a fluorescence correlation spectroscopy study reveals NC tends to form a quasi-stable complex with a scavenger molecule, ensuring charge transfer completed (τct ≈ 0.6 ps) much before the complex breaks apart (>600 μs). The overall results of our study highlight the promise shown by 12-faceted NCs and their implications in modern applications, including hot carrier solar cells.

Copper Nanoclusters as an Effective Enzyme Inhibitor on the Activity Modulation of α-Chymotrypsin

The present work is undertaken with an objective to find out the suitability of CuNCs on the activity modulation of a model enzyme, α chymotrypsin (α-ChT). This work also aims to highlight the role of surface chemistry corresponding to CuNCs in modulating the activity of α-ChT. For this purpose, two different types of CuNCs having chemically different surface ligands, namely, cysteine (Cys) and tannic acid (TA), have been synthesized. Subsequently, the interaction of these CuNCs with α-ChT has been investigated by employing various spectroscopic techniques at both ensemble average and single molecule level. Results obtained from enzyme kinetics studies have revealed that both the CuNCs act as good enzyme inhibitors. While Cys-CuNCs almost completely diminish the activity of α-ChT through a competitive inhibition mechanism, TA-CuNCs partially reduce the enzyme activity through a noncompetetive inhibition mechanism indicating the vital role of surface ligand in the regulation of α-ChT activity. To gain a molecular level understanding of the enzyme–inhibitor interaction event, fluorescence spectroscopy, ITC measurements, fluorescence correlation spectroscopy (FCS), agarose gel electrophoresis, and circular dichroism (CD) spectroscopy are conducted. Thermodynamics results obtained from fluorescence titration experiment and ITC measurements have indicated that Cys-CuNCs follow a one-step binding process, whereas TA-CuNCs follow a two-step binding process. Moreover, FCS studies have provided evidence for the interaction of CuNCs with α-ChT at the single molecule level. Importantly, circular dichroism (CD) measurements have demonstrated that the basic structure of α-ChT remains almost unaltered in the presence of CuNCs. The outcome of the present study is expected to open up a possibility of using CuNCs as an effective nanoscale enzyme regulator for various biological applications.

Fluorescence correlation spectroscopy studies of antimicrobial peptide binding to phospholipid vesicles.

Antimicrobial peptides (AMPs) perform an essential part of the innate immune defence of all organisms and are potential candidates for last-resort antibiotics. Aurein 1.2 is a natural AMP from the skin secretions of the Australian bell frog. The molecular details underpinning the mechanism of action, specificity and selectivity of this AMP are mostly unknown. We used fluorescence correlation spectroscopy (FCS) to assess the binding and membrane modulating properties of fluorescently labeled Aurein 1.2 and its specific mutants with lipid vesicles of different physicochemical characteristics. Our results indicate that aurein 1.2 binds non-specifically to all kinds of membranes; however, the presence of specific lipid species can prevent membrane disruption. We note the general absence of “carpet-like” disruption patterns; instead, a strong membrane modulating activity was observed. We confirmed that the FCS technique produces accurate binding measurements for nanomolar peptide concentrations, which is almost impossible with other techniques for studying the interaction of AMPs with lipid membranes. We suggest that our results could guide the design of membrane-active AMPs.

Photoluminescence Blinking Revealing Static and Dynamic Heterogeneity of the Hole Transfer Process in Phenothiazine-Adsorbed FAPbBr3 Single Nanocrystals

Lead halide perovskites are perhaps the most important substances for solar photovoltaic applications in recent times. The extraction or transfer of the electrons and holes produced on photoexcitation of these perovskites is a key process in this regard. Here, we investigate how hole transfer from FAPbBr3 to adsorbed phenothiazine (PTZ) influences the photoluminescence (PL) blinking dynamics of FAPbBr3-PTZ single nanocrystals (NCs) in immobilized and fluid conditions. In the immobilized state, while the single FAPbBr3 NCs show two-state (on–off) blinking, the FAPbBr3-PTZ single NCs exhibit a complex blinking pattern with several levels of PL intensity due to the hole transfer process. Analysis of the PL blinking data reveals the dispersive/heterogeneous nature of the hole transfer dynamics. Both particle-to-particle variation and time-dependent variation of the hole transfer rate of any given single NC are observed. Fluorescence correlation spectroscopy studies in the fluid state confirm this dispersive nature of the hole transfer dynamics, indicating the formation of a short-lived (<100 ns) charge-separated state, FAPbBr3–-PTZ+.

Energy Landscapes for Base-Flipping in a Model DNA Duplex.

We explore the process of base-flipping for four central bases, adenine, guanine, cytosine, and thymine, in a deoxyribonucleic acid (DNA) duplex using the energy landscape perspective. NMR imino-proton exchange and fluorescence correlation spectroscopy studies have been used in previous experiments to obtain lifetimes for bases in paired and extrahelical states. However, the difference of almost 4 orders of magnitude in the base-flipping rates obtained by the two methods implies that they are exploring different pathways and possibly different open states. Our results support the previous suggestion that minor groove opening may be favored by distortions in the DNA backbone and reveal links between sequence effects and the direction of opening, i.e., whether the base flips toward the major or the minor groove side. In particular, base flipping along the minor groove pathway was found to align toward the 5′ side of the backbone. We find that bases align toward the 3′ side of the backbone when flipping along the major groove pathway. However, in some cases for cytosine and thymine, the base flipping along the major groove pathway also aligns toward the 5′ side. The sequence effect may be caused by the polar interactions between the flipping-base and its neighboring bases on either of the strands. For guanine flipping toward the minor groove side, we find that the equilibrium constant for opening is large compared to flipping via the major groove. We find that the estimated rates of base opening, and hence the lifetimes of the closed state, obtained for thymine flipping through small and large angles along the major groove differ by 6 orders of magnitude, whereas for thymine flipping through small angles along the minor groove and large angles along the major groove, the rates differ by 3 orders of magnitude.

Preferential Binding of Epirubicin Hydrochloride with Single Nucleotide Mismatched DNA and Subsequent Sequestration by a Mixed Micelle.

Targeting mismatched base pairs containing DNA using small molecules and exploring the underlying mechanism involved during the binding interactions is one of the fundamental aspects of drug design. These molecules in turn are used in nucleic acid targeted therapeutics and cancer diagnosis. In this work, we systematically delineate the binding of the anticancer drug, epirubicin hydrochloride (EPR) with 20-mer duplex DNA, having both natural nucleobase pairing and thermodynamically least stable non-Watson-Crick base pairing. From the thermal denaturation studies, we observed that EPR can remarkably enhance the thermal stability of cytosine-cytosine (CC) and cytosine-thymine (CT) mismatched (MM) DNA over other 20-mer duplex DNA. From steady-state fluorescence spectroscopy and isothermal titration calorimetry studies, we concluded that EPR binds strongly with the mismatched duplex DNA through the intercalation binding mode. The interaction of EPR and duplex DNA has also been monitored at a single molecular resolution using fluorescence correlation spectroscopy (FCS). Dynamic quantitates such as diffusion coefficients and hydrodynamic radii obtained from an FCS study along with association and dissociation rate constants estimated from intensity time trace analyses further substantiate the stronger binding affinity of EPR to the thermally less stable mismatched DNA, formed by the most discriminating nucleobase (viz. cytosine). Additionally, we have shown that EPR can be sequestered from nucleic acids using a mixed micellar system of an anionic surfactant and a triblock copolymer. From thermal denaturation studies and circular dichroism spectroscopy, we found that the extent of drug sequestration depends on the binding affinity of EPR to the duplex DNA, and this mixed micellar system can be employed for the removal of excess drug in the case of a drug overdose.

Biophysical properties of the isolated spike protein binding helix of human ACE2

The entry of the severe acute respiratory syndrome coronavirus 2 virus in human cells is mediated by the binding of its surface spike protein to the human angiotensin-converting enzyme 2 (ACE2) receptor. A 23-residue long helical segment (SBP1) at the binding interface of human ACE2 interacts with viral spike protein and therefore has generated considerable interest as a recognition element for virus detection. Unfortunately, emerging reports indicate that the affinity of SBP1 to the receptor-binding domain of the spike protein is much lower than that of the ACE2 receptor itself. Here, we examine the biophysical properties of SBP1 to reveal factors leading to its low affinity for the spike protein. Whereas SBP1 shows good solubility (solubility > 0.8 mM), circular dichroism spectroscopy shows that it is mostly disordered with some antiparallel β-sheet content and no helicity. The helicity is substantial (>20%) only upon adding high concentrations (≥20% v/v) of 2,2,2-trifluoroethanol, a helix promoter. Fluorescence correlation spectroscopy and single-molecule photobleaching studies show that the peptide oligomerizes at concentrations >50 nM. We hypothesized that mutating the hydrophobic residues (F28, F32, and F40) of SBP1, which do not directly interact with the spike protein, to alanine would reduce peptide oligomerization without affecting its spike binding affinity. Whereas the mutant peptide (SBP1mod) shows substantially reduced oligomerization propensity, it does not show improved helicity. Our study shows that the failure of efforts, so far, to produce a short SBP1 mimic with a high affinity for the spike protein is not only due to the lack of helicity but is also due to the heretofore unrecognized problem of oligomerization.

Probing How Various Metal Ions Interact with the Surface of QDs: Implication of the Interaction Event on the Photophysics of QDs.

With an aim to understand the mechanism of interaction between quantum dots (QDs) and various metal ions, fluorescence response of less-toxic and water-soluble glutathione-capped Zn-Ag-In-S (GSH@ZAIS) QDs in the presence of different metal ions has been investigated at both ensemble and single-molecule level. Fourier transform infrared (FT-IR) spectroscopy has also been performed to obtain a molecular level understanding of the interaction event. The steady-state data reveal no significant change in QD emission for alkali and alkaline earth metal ions, while there is a decrease in fluorescence intensity for transition metal (TM) and some heavy transition metal (HTM) ions. Interestingly, a significant fluorescent enhancement (FE) (19-96%) of QDs is found for Cd2+ ions. Time-resolved fluorescence studies reveal that all the three decay components of QDs decrease in the presence of first-row TM ions. However, in the case of Cd2+, the shorter component is found to increase while the longer one decreases. The analysis of data reveals that photoinduced electron transfer is responsible for fluorescence quenching of QDs in the presence of first-row TM ions and destruction/removal of trap/defect states in the case of Cd2+ causes the FE. In FT-IR experiments, a prominent peak at 670 cm-1, corresponding to Cd-S stretching vibrations, indicates strong ground-state interactions between the -SH of GSH and Cd2+ ions. Moreover, a decrease in the diffusion coefficient of QDs in the presence of Cd2+ ions during fluorescence correlation spectroscopy (FCS) studies further substantiates the removal of GSH by Cd2+ from the surface of QDs. The optical output of this study demonstrates that ZAIS can be used for fluorescence signaling of various metal ions and in particular selective detection of Cd2+. More importantly, these results also suggest that Cd2+ can effectively be used for enhancing the fluorescence quantum yield of thiol-capped QDs such as GSH@ZAIS.

Structural Stability and Conformational Dynamics of Cytochrome c in Hydrated Deep Eutectic Solvents.

Many deep eutectic solvents (DESs) are currently being explored as environment-friendly media for biorelated applications. As an understanding of the effect of these solvents on the structure of biomolecules is crucial for these applications, we study how two DESs comprising trimethylglycine (TMG) and ethylene glycol (EG) or glycerol (GL) influence the structural stability and conformational dynamics of cytochrome c (Cytc) using single-molecule-based fluorescence correlation spectroscopy (FCS) technique and several other ensemble-based biophysical methods. The FCS studies on A488-labeled Cytc enable an estimation of the size (20.5 ± 1.5 Å) of the protein and capture its conformational dynamics (54 ± 2 μs) in aqueous buffered solution. It is observed that both size and conformational dynamics of the protein are influenced in the presence of the DESs, but this effect is more pronounced in the case of TMG-EG. The ensemble measurements on both labeled and wild-type Cytc reveal that the protein structure is unfolded completely by TMG-EG, whereas the structure is slightly altered by TMG-GL. The results suggest that the behavior of Cytc in hydrated DESs is determined by the strength of interactions between the DES constituents as well as that between the constituents and the water molecules present in the system.

Effects of Viscosity and Refractive Index on the Emission and Diffusion Properties of Alexa Fluor 405 Using Fluorescence Correlation and Lifetime Spectroscopies

Fluorescence Correlation Spectroscopy (FCS) studies of the interaction of polymers or proteins in solution are strongly affected by the viscosity and refractive index of the medium, and the effects are likely to be more significant with the use of short wavelength excitation (e.g., 405 nm diode lasers). Failing to account for these issues can lead to incorrect measurement of average size, conformational changes, and dynamic behaviour of polymers and proteins. Steady-state, time-resolved, and FCS measurements of Alexa 405 in glycerol:water mixtures were performed to determine its suitability for FCS measurements with 405 nm excitation. The effects of the refractive index and viscosity on the diffusion coefficient and photophysical parameters (lifetime and relative quantum yield) of the fluorophore were determined. Alexa 405 lifetime decreased from 3.55 ns in water to 3.25 ns in a 50:50 glycerol:water mixture, while its diffusion coefficient dropped from 333 ± 16 to 44 ± 1 µm2s- 1. Lifetime data collected from micromolar solutions of Alexa 405 did however also suggest that as solvent polarity decreased, aggregates (excimers) were formed as evidenced by the appearance of a rising edge in the decay plots. The interdependence between lifetime, refractive index, and diffusion coefficient could be accurately fitted by a simple polynomial function indicating that the probe is well behaved and predictable in the glycerol:water model system. Overall, Alexa 405 is a most promising and reliable probe for FCS measurement using violet laser diode excitation sources.

Efficient G protein coupling is not required for agonist-mediated internalization and membrane reorganization of the adenosine A3 receptor.

Organization of G protein‐coupled receptors at the plasma membrane has been the focus of much recent attention. Advanced microscopy techniques have shown that these receptors can be localized to discrete microdomains and reorganization upon ligand activation is crucial in orchestrating their signaling. Here, we have compared the membrane organization and downstream signaling of a mutant (R108A, R3.50A) of the adenosine A3 receptor (A3AR) to that of the wild‐type receptor. Fluorescence Correlation Spectroscopy (FCS) studies with a fluorescent agonist (ABEA‐X‐BY630) demonstrated that both wild‐type and mutant receptors bind agonist with high affinity but in subsequent downstream signaling assays the R108A mutation abolished agonist‐mediated inhibition of cAMP production and ERK phosphorylation. In further FCS studies, both A3AR and A3AR R108A underwent similar agonist‐induced increases in receptor density and molecular brightness which were accompanied by a decrease in membrane diffusion after agonist treatment. Using bimolecular fluorescence complementation, experiments showed that the R108A mutant retained the ability to recruit β‐arrestin and these receptor/arrestin complexes displayed similar membrane diffusion and organization to that observed with wild‐type receptors. These data demonstrate that effective G protein signaling is not a prerequisite for agonist‐stimulated β‐arrestin recruitment and membrane reorganization of the A3AR.

Probing the Interaction of Bovine Serum Albumin with Copper Nanoclusters: Realization of Binding Pathway Different from Protein Corona

With an aim to understand the interaction mechanism of bovine serum albumin (BSA) with copper nanoclusters (CuNCs), three different types CuNCs having chemically different surface ligands, namely, tannic acid (TA), chitosan, and cysteine (Cys), have been fabricated, and investigations are carried out in the absence and presence of protein (BSA) at ensemble-averaged and single-molecule levels. The CuNCs, capped with different surface ligands, are consciously chosen so that the role of surface ligands in the overall protein-NCs interactions is clearly understood, but, more importantly, to find whether these CuNCs can interact with protein in a new pathway without forming the "protein corona", which otherwise has been observed in relatively larger nanoparticles when they are exposed to biological fluids. Analysis of the data obtained from fluorescence, ζ-potential, and ITC measurements has clearly indicated that the BSA protein in the presence of CuNCs does not attain the binding stoichiometry (BSA/CuNCs > 1) that is required for the formation of "protein corona". This conclusion is further substantiated by the outcome of the fluorescence correlation spectroscopy (FCS) study. Further analysis of data and thermodynamic calculations have revealed that the surface ligands of the CuNCs play an important role in the protein-NCs binding events, and they can alter the mode and thermodynamics of the process. Specifically, the data have demonstrated that the binding of BSA with TA-CuNCs and Chitosan-CuNCs follows two types of binding modes; however, the same with Cys-CuNCs goes through only one type of binding mode. Circular dichroism (CD) measurements have indicated that the basic structure of BSA remains almost unaltered in the presence of CuNCs. The outcome of the present study is expected to encourage and enable better application of NCs in biological applications.

Mechanistic Investigation of the Defect Activity Contributing to the Photoluminescence Blinking of CsPbBr3 Perovskite Nanocrystals.

Exploration of the full potential of the perovskite nanocrystals (NCs) for different applications requires a thorough understanding of the pathways of recombination of the photogenerated charge carriers and associated dynamics. In this work, we have tracked the recombination routes of the charge carriers by probing photoluminescence (PL) intermittency of the immobilized and freely diffusing single CsPbBr3 NCs employing a time-tagged-time-resolved method. The immobilized single CsPbBr3 NCs show a complex PL time-trace, a careful analysis of which reveals that nonradiative band-edge recombination through trap states, trion recombination, and trapping of the hot carriers contribute to the blinking behavior of any given NC. A drastically suppressed PL blinking observed for the NCs treated with a tetrafluoroborate salt indicates elimination of most of the undesired recombination processes. A fluorescence correlation spectroscopy (FCS) study on the freely diffusing single NCs shows that enhanced PL and suppressed blinking of the treated particles are the outcome of an increase in per-particle brightness, not due to any increase in the number of particles undergoing "off"-"on" transition in the observation volume. The mechanistic details obtained from this study on the origin of blinking in CsPbBr3 NCs provide deep insight into the radiative and nonradiative charge carrier recombination pathways in these important materials, and this knowledge is expected to be useful for better design and development of bright photoluminescent samples of this class for optoelectronic applications.

Small Molecule Adsorption on Colloidal Alumina and Zirconia Abrasives Used in Chemical-Mechanical Planarization (CMP) Slurries

Small molecule adsorption on CMP slurry abrasive particles has been investigated previously for silica (1) and ceria (2, 3) abrasives using fluorescence correlation spectroscopy (FCS) and attenuated total reflectance – Fourier transform infrared spectroscopy (ATR-FTIR). The need for further characterization of such adsorptive interactions is dictated by the increasing use of abrasive particles in CMP slurries with minimized hydrodynamic diameters (≤ 10 nm) as a means to reduce surface defects created during CMP processes. Incorporation of ever-smaller abrasive particles in CMP slurries will, however, promote the adsorption of chemical additives on a slurry’s abrasive particles due to the increase in the total abrasive particle surface area. In the present study, FCS and ATR-FTIR are employed in the analysis of small molecule adsorption on colloidal alumina and zirconia abrasive particles. This work is motivated by the widespread commercial use of alumina-based slurries in CMP processes for planarization of deposited copper films on dielectric layers and the proposed use (4) of zirconia abrasives in CMP slurries for metal film removal on dielectric metal oxide materials. The reported FCS studies use fluorescent dyes as probes for adsorption sites on alumina and zirconia colloids dispersed in aqueous solution. Described ATR-FTIR analyses summarize the characterization of molecular interactions driving glycine and picolinic acid adsorption on porous thin films of colloidal alumina and zirconia, respectively. The potential for employing these methods in studies modelling the adsorption of CMP slurry additives on a metal film deposited on a silicon wafer surface is also discussed.