A Novel Quantum Dot-Based pH Probe for Long-Term Fluorescence Lifetime Imaging Microscopy Experiments in Living Cells

Monitoring cancer treatments in melanoma cells using a fluorescence lifetime nanoprobe based on a CdSe/ZnS quantum dot functionalized with a peptide containing D-penicillamine and histidine

Anticancer therapies with cisplatin and volasertib (BI-6727) were monitored by fluorescence lifetime imaging microscopy (FLIM) in live SK-Mel-2 melanoma cells. A CdSe/ZnS quantum dot functionalized with a peptide containing D-penicillamine and histidine (CdSe/ZnS-PH) was used as intracellular pH fluorescent probe. A faster cytosol acidification was observed for cells treated with cisplatin when compared to volasertib. The first changes in the intracellular pH were found after 2 hours of treatment with cisplatin and 8 hours with volasertib. Additionally, the relationship between cytosol acidification and apoptosis was investigated using an innovative methodology based on time-resolved fluorescence measurements. Similar low percentages of apoptotic cells were observed after short incubation periods (2 - 8 hours) with both drugs. In contrast, late apoptosis and death were found for a large fraction of cells during 24-hour incubation with cisplatin but not volasertib. Thus, the early acidification observed in cisplatin treatment could accelerate apoptosis and cell death. Despite volasertib treatment shows slower mechanism of action than cisplatin, similar inhibitory effects were found for both drugs at longer incubation periods (72 hours). This study proves the potential of CdSe/ZnS-PH nanoparticle as a fluorescence lifetime probe in the study of the mechanism of action of antitumor drugs.

Self-Reporting Therapeutic Protein Nanoparticles

We present a modular strategy to synthesize nanoparticle sensors equipped with dithiomaleimide-based, fluorescent molecular reporters capable of discerning minute changes in interparticle chemical environments based on fluorescence lifetime analysis. Three types of nanoparticles were synthesized with the aid of tailor-made molecular reporters, and it was found that protein nanoparticles exhibited greater sensitivity to changes in the core environment than polymer nanogels and block copolymer micelles. Encapsulation of the hydrophobic small-molecule drug paclitaxel (PTX) in self-reporting protein nanoparticles induced characteristic changes in fluorescence lifetime profiles, detected via time-resolved fluorescence spectroscopy. Depending on the mode of drug encapsulation, self-reporting protein nanoparticles revealed pronounced differences in their fluorescence lifetime signatures, which correlated with burst- vs diffusion-controlled release profiles observed in previous reports. Self-reporting nanoparticles, such as the ones developed here, will be critical for unraveling nanoparticle stability and nanoparticle-drug interactions, informing the future development of rationally engineered nanoparticle-based drug carriers.

Fluorescent Nanodiamonds for High-Resolution Thermometry in Biology

Optically active color centers in diamond and nanodiamonds can be utilized as quantum sensors for measuring various physical parameters, particularly magnetic and electric fields, as well as temperature. Due to their small size and possible surface functionalization, fluorescent nanodiamonds are extremely attractive systems for biological and medical applications since they can be used for intracellular experiments. This review focuses on fluorescent nanodiamonds for thermometry with high sensitivity and a nanoscale spatial resolution for the investigation of living systems. The current state of the art, possible further development, and potential limitations of fluorescent nanodiamonds as thermometers will be discussed here.

Automatic segmentation of lysosomes and analysis of intracellular pH with Radachlorin photosensitizer and FLIM

We report application of the fluorescence lifetime imaging microscopy (FLIM) for analysis of distributions of intracellular acidity using a chlorin-e6 based photosensitizer Radachlorin. An almost two-fold increase of the photosensitizer fluorescence lifetime in alkaline microenvironments as compared to acidic ones allowed for clear distinguishing between acidic and alkaline intracellular structures. Clusterization of a phasor plot calculated from fits of the FLIM raw data by two Gaussian distributions provided accurate automatic segmentation of lysosomes featuring acidic contents. The approach was validated in colocalization experiments with LysoTracker fluorescence in living cells of four established lines. The dependence of photosensitizer fluorescence lifetime on microenvironment acidity allowed for estimation of pH inside the cells, except for the nuclei, where photosensitizer does not penetrate. The developed method is promising for combined application of the photosensitizer for both photodynamic treatment and diagnostics.

Interactions between Quantum Dots and G-Actin

Quantum dots (QDs) are a type of nanoparticle with excellent optical properties, suitable for many optical-based biomedical applications. However, the potential of quantum dots to be used in clinical settings is limited by their toxicity. As such, much effort has been invested to examine the mechanism of QDs' toxicity. Yet, the current literature mainly focuses on ROS- and apoptosis-mediated cell death induced by QDs, which overlooks other aspects of QDs' toxicity. Thus, our study aimed to provide another way by which QDs negatively impact cellular processes by investigating the possibility of protein structure and function modification upon direct interaction. Through shotgun proteomics, we identified a number of QD-binding proteins, which are functionally associated with essential cellular processes and components, such as transcription, translation, vesicular trafficking, and the actin cytoskeleton. Among these proteins, we chose to closely examine the interaction between quantum dots and actin, as actin is one of the most abundant proteins in cells and plays crucial roles in cellular processes and structural maintenance. We found that CdSe/ZnS QDs spontaneously bind to G-actin in vitro, causing a static quenching of G-actin's intrinsic fluorescence. Furthermore, we found that this interaction favors the formation of a QD-actin complex with a binding ratio of 1:2.5. Finally, we also found that CdSe/ZnS QDs alter the secondary structure of G-actin, which may affect G-actin's function and properties. Overall, our study provides an in-depth mechanistic examination of the impact of CdSe/ZnS QDs on G-actin, proposing that direct interaction is another aspect of QDs' toxicity.

Effect of trifluoroacetic acid on InP/ZnSe/ZnS quantum dots: mimicking the surface trap and their effects on the photophysical properties

Understanding the precise effects of defects on the photophysical properties of quantum dots (QDs) is essential to their development with near-unity luminescence. Because of the complicated nature of defects in QDs, the origins and detailed roles of the defects still remain rarely understood. In this regard, we used detailed chemical analysis to investigate the effect of surface defects on the optical properties of InP/ZnSe/ZnS QDs by introducing shell defects through controlled trifluoroacetic acid (TFA) etching. TFA treatment on the InP/ZnSe/ZnS QDs partially removed the ZnS shell as well as ligands and reduced the quantum yield by generating energetically deep surface traps. The surface defects of QDs by TFA cause charged trap sites inducing an Auger recombination process with a rate of ca. 200 ps. Based on these results, we proposed possible trap-assisted non-radiative decay pathways between the band-edge state and surface deep traps in InP/ZnSe/ZnS QDs.

Nanoparticles for Interrogation of Cell Signaling

Cell functions rely on signal transduction-the cascades of molecular interactions and biochemical reactions that relay extracellular signals to the cell interior. Dissecting principles governing the signal transduction process is critical for the fundamental understanding of cell physiology and the development of biomedical interventions. The complexity of cell signaling is, however, beyond what is accessible by conventional biochemistry assays. Thanks to their unique physical and chemical properties, nanoparticles (NPs) have been increasingly used for the quantitative measurement and manipulation of cell signaling. Even though research in this area is still in its infancy, it has the potential to yield new, paradigm-shifting knowledge of cell biology and lead to biomedical innovations. To highlight this importance, we summarize in this review studies that pioneered the development and application of NPs for cell signaling, from quantitative measurements of signaling molecules to spatiotemporal manipulation of cell signal transduction.

Analysis of Radachlorin localization in living cells by fluorescence lifetime imaging microscopy

Intracellular localization of photosensitizer molecules is influential on cell death pathway at photodynamic treatment and is thus an important aspect in achieving enhanced efficacy of photodynamic therapy. In this paper we performed thorough studies of the distribution of Radachlorin photosensitizer in three established cell lines: HeLa, A549, and 3T3 with fluorescence lifetime imaging microscopy through the analysis of lifetime distributions. Experiments carried out in Radachlorin solutions in phosphate buffered saline revealed the pronounced dependence of the fluorescence quantum yield and lifetime on solution pH. This finding was used for analysis of lifetime images of living cells and their phasor plot representations and allowed us to suggest that Radachlorin localized predominantly in lysosomes, known to have acidic pH values. Experiments on co-localization of Radachlorin fluorescence lifetimes and LysoTracker fluorescence intensity supported this suggestion. The results obtained show that the inhomogeneity of fluorescence quantum yield within a cell can be significant due to lower pH values in lysosomes than in other intracellular compartments. This finding suggests that the actual amount of accumulated Radachlorin can be underestimated if being evaluated solely by comparison of fluorescence intensities.

Parental exposure to CdSe/ZnS QDs affects cartilage development in rare minnow (Gobiocypris rarus) offspring

Cartilage development is a sensitive process that is easily disturbed by environmental toxins. In this study, the toxicity of CdSe/ZnS quantum dots on the skeleton of the next generation (F1) was evaluated using rare minnows (Gobiocypris rarus) as model animals. Four-month-old sexually mature parental rare minnows (F0) were selected and treated with 0, 100, 200, 400 and 800 nmol/L CdSe/ZnS quantum dots for 4 days. Embryos of F1 generation rare minnows were obtained by artificial insemination. The results showed that with increasing maternal quantum dots exposure, the body length of F1 embryos decreased, the overall calcium content decreased, and the deformity and mortality rates increased. Alcian blue staining results showed that the lengths of the craniofacial mandible, mandibular arch length, mandibular width, and CH-CH and CH-PQ angles of larvae of rare minnows increased; histological hematoxylin-eosin staining further indicated that quantum dots affected the development of chondrocytes. Furthermore, high concentrations of CdSe/ZnS quantum dots inhibited the transcript expression of the bmp2b, bmp4, bmp6, runx2b, sox9a, lox1 and col2α1 genes. In conclusion, CdSe/ZnS quantum dots can affect the skeletal development of F1 generation embryos of rare minnows at both the individual and molecular levels, the damage to the craniofacial bone is more obvious, and the toxic effect of high concentrations of quantum dots (400 nmol/L and 800 nmol/L) is more significant.

Fine Tuning of Multiphoton AIE Emission Behavior, Organelle Targeting, and Fluorescence Lifetime Imaging of Terpyridine Derivatives by Alkyl Chain Engineering

In this work, a series of multiphoton terpyridine agents (ZA, ZA-Mex, and ZA-Hex) for fluorescence lifetime imaging microscopy (FLIM) are designed and synthesized. The results from photophysical property research reveal that ZA-Hex, as an N-hexylated terpyridine salt, has stronger three-photon aggregation-induced emission (AIE) properties compared to ZA-Mex due to enhanced intramolecular charge transfer (ICT) performance. All three terpyridine derivatives possess suitable fluorescence intensities and stable fluorescence lifetimes under different pH conditions (pH = 4.0-8.0), thereby performing multiphoton fluorescence lifetime imaging. For biological imaging applications, it is found that ZA shows good lipid droplet (LD) turn-on fluorescence performance, and ZA-Hex could easily accumulate in mitochondria with high specificity. This is the first report of terpyridine salts as three-photon AIE probes used for multiphoton FLIM imaging.