Photoinduced enhancement in the luminescence of hydrophilic quantum dots coated with photocleavable ligands

Light-Triggered Switching of Quantum Dot Photoluminescence through Excited-State Electron Transfer to Surface-Bound Photochromic Molecules

This paper describes reversible "on-off" switching of the photoluminescence (PL) intensity of CdSe quantum dots (QDs), mediated by photochromic furylfulgide carboxylate (FFC) molecules chemisorbed to the surfaces of the QDs. Repeated cycles of UV and visible illumination switch the FFC between "closed" and "open" isomers. Reversible switching of the QDs' PL intensity by >80% is enabled by different rates and yields of PL-quenching photoinduced electron transfer (PET) from the QDs to the respective isomers. This difference is consistent with cyclic voltammetry measurements and density functional calculations of the isomers' frontier orbital energies. This work demonstrates fatigue-resistant modulation of the PL of a QD-molecule complex through remote control of PET. Such control potentially enables applications, such as all-optical memory, sensing, and imaging, that benefit from a fast, tunable, and reversible response to light stimuli.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Silk Fluorescence Collimator for Ultrasensitive Humidity Sensing and Light-Harvesting in Semitransparent Dye-Sensitized Solar Cells

This work examines the self-collimation effect of silk materials on fluorescence emission/detection. A macroscopic regulation strategy, coupled with meso-reconstruction and meso-functionalization, is adopted to amplify the fluorescence emission of organic fluorescent dyes (i.e., Rhodamine 6G (R6G)) using silk photonic crystal (PC) films. The fluorescence emission can be linearly enhanced or inhibited by a PC as a result of the photonic bandgap coupling with the excitation light and/or emission light. Depending on the design of the silk fluorescence collimator, the emission can reach 49.37 times higher than the control. The silk fluorescence collimator can be applied to achieve significant benefits: for instance, as a humidity sensor, it provides good reproducibility and a sensitivity of 28.50 a.u./% relative humidity, which is 80.78 times higher than the sensitivity of the control, and as a novel curtain, it raises the energy conversion efficiency of the semitransparent dye-sensitized solar cells (DSSCs) by 16%.

Molecular imaging with nanoparticles: the dwarf actors revisited 10 years later

We explore present-day trends and challenges in nanomedicine. Creativity in the laboratories continues: the published literature on novel nanoparticles is now vast. Nanoagents are discussed here which are composed entirely of strongly photoluminescent materials, tunable to desired optical properties and of inherently low toxicity. We focus on "quantum nanoparticles" prepared from allotropes of carbon. The principles behind strong, tunable photoluminescence are quantum mechanical: we present them in simple outline. The major industries racing to develop these materials can offer significant technical guidance to nanomedicine, which could help to custom-design strongly signalling nanoagents specifically for stated clinical applications. Since such agents are small, they can be targeted easily, making active targeting possible. We consider it timely now to study the interactions nanoparticles undergo with tissue components in living animals and to learn to understand and overcome the numerous barriers the organism interposes between the blood and targets in or on parenchymal cells. As the near infra-red spectrum opens up, detection of glowing nanoparticles several centimeters deep in a living human subject becomes calculable and we present a simple way to do this. Finally, we discuss the slow-fuse and resource-inefficient entry of nanoparticles into clinical application. A first possible reason is failure to target across the body's barriers, see above. Second, in the sparse translational landscape funding and support gaps yawn widely between academic research and subsequent development. We consider the agendas of the numerous "stakeholders" participating in this sad landscape and point to some faint glimmers of hope for the future.

Quantum Dot-Dye Conjugates for Biosensing, Imaging, and Therapy

Adding value to the intrinsic properties of quantum dots (QDs), a strategy to conjugate dyes on the surface of QDs offers new opportunities, since the coupling between QD and dyes can be designed to allow Förster resonance energy transfer (FRET) and/or electron transfer (eT). These processes are accompanied by the change of QD and/or dye fluorescence and subsequent photochemical reactions (e.g., generation of 1 O2 ). Based on the change of fluorescence signals by the interaction with biomolecules, QD-dye conjugates are exploited as biosensors for the detection of pH, O2 , nicotinamide adenine dinucleotide (phosphate), ions, proteases, glutathione, and microRNA. QD-dye conjugates also can be modulated by the irradiation of external light; this concept is demonstrated for fluorescence super-resolution imaging as photoactivatable or photoswitchable probes. When QDs are conjugated with photosensitizing dyes, the QD-dye conjugates can generate 1 O2 in a repetitive manner for better cancer treatment, and can also be available for approaches using two-photon excitation or bioluminescence resonance energy transfer mechanisms for deep tissue imaging. Here, the recent advances in QD-dye conjugates, where FRET or eT produces fluorescence readouts or photochemical reactions, are reviewed. Various QD-dye conjugate systems and their biosensing/imaging and photodynamic therapeutics are summarized.

Light-Induced Fluorescence Modulation of Quantum Dot-Crystal Violet Conjugates: Stochastic Off-On-Off Cycles for Multicolor Patterning and Super-Resolution

Photoswitching or modulation of quantum dots (QDs) can be promising for many fields that include display, memory, and super-resolution imaging. However, such modulations have mostly relied on photomodulations of conjugated molecules in QD vicinity, which typically require high power of high energy photons at UV. We report a visible light-induced facile modulation route for QD-dye conjugates. QD crystal violets conjugates (QD-CVs) were prepared and the crystal violet (CV) molecules on QD quenched the fluorescence efficiently. The fluorescence of QD-CVs showed a single cycle of emission burst as they go through three stages of (i) initially quenched "off" to (ii) photoactivated "on" as the result of chemical change of CVs induced by photoelectrons from QD and (iii) back to photodarkened "off" by radical-associated reactions. Multicolor on-demand photopatterning was demonstrated using QD-CV solid films. QD-CVs were introduced into cells, and excitation with visible light yielded photomodulation from "off" to "on" and "off" by nearly ten fold. Individual photoluminescence dynamics of QD-CVs was investigated using fluorescence correlation spectroscopy and single QD emission analysis, which revealed temporally stochastic photoactivations and photodarkenings. Exploiting the stochastic fluorescence burst of QD-CVs, simultaneous multicolor super-resolution localizations were demonstrated.

Photocontrollable Intermittent Release of Doxorubicin Hydrochloride from Liposomes Embedded by Azobenzene-Contained Glycolipid

Azobenzene-contained glycolipids GlyAzoCns, newly structured azobenzene derivatives, which have an azobenzene moiety between the galactosyl and carbon chains of various sizes, have been synthesized. The GlyAzoCns undergo reversible photoinduced isomerization in both ethanol solution (free state) and liposomal bilayer (restricted state) upon irradiation with UV and vis light alternately. The drug release of Liposome@Gly induced by isomerization was found to be an instantaneous behavior. The photoinduced control of DOX release from liposome was investigated in various modes. The Liposome@Glys have been found to keep the entrapped DOX stably in the dark with less than 10% leakage in 10 h but release nearly 100% of cargos instantaneously with UV irradiation. The molecular structure of GlyAzoCns and the property of the liposomal bilayer were considered as important factors influencing drug release. Among the synthesized GlyAzoCns, GlyAzoC7 was shown to be the most efficient photosensitive actuator for controlling drug release. A lower proportion of cholesterol in Liposome@Glys was conducive to promote the release amount. Results indicated that the synthesized GlyAzoCns could act as a role of smart actuators in the liposome bilayer and control the drug to release temporarily and quantitatively.

A Thioacetal Photocage Designed for Dual Release: Application in the Quantitation of Therapeutic Release by Synchronous Reporter Decaging

Despite the immense potential of existing photocaging technology, its application is limited by the paucity of advanced caging tools. Here, we report on the design of a novel thioacetal ortho-nitrobenzaldehyde (TNB) dual arm photocage that enabled control of the simultaneous release of two payloads linked to a single TNB unit. By using this cage, which was prepared in a single step from commercial 6-nitroverataldehyde, three drug-fluorophore conjugates were synthesized: Taxol-TNB-fluorescein, Taxol-TNB-coumarin, and doxorubicin-TNB-coumarin, and long-wavelength UVA light-triggered release experiments demonstrated that dual payload release occurred with rapid decay kinetics for each conjugate. In cell-based assays performed in vitro, dual release could also be controlled by UV exposure, resulting in increased cellular fluorescence and cytotoxicity with potency equal to that of unmodified drug towards the KB carcinoma cell line. The extent of such dual release was quantifiable by reporter fluorescence measured in situ and was found to correlate with the extent of cytotoxicity. Thus, this novel dual arm cage strategy provides a valuable tool that enables both active control and real-time monitoring of drug activation at the delivery site.

Surface plasmon inhibited photo-luminescence activation in CdSe/ZnS core-shell quantum dots

In a composite film of Cd x  Se y  Zn1-x  S1-y gradient core-shell quantum dots (QDs) and gold nanorods (NRs), the optical properties of the QDs are drastically affected by the plasmonic nanoparticles. We provide a careful study of the two-step formation of the film and its morphology. Subsequently we focus on QD luminescence photoactivation-a process induced by photochemical changes on the QD surface. We observe that even a sparse coverage of AuNRs can completely inhibit the photoactivation of the QDs' emission in the film. We demonstrate that the inhibition can be accounted for by a rapid energy transfer between QDs and AuNRs. Finally, we propose that the behavior of emission photoactivation can be used as a signature to distinguish between energy and electron transfer in the QD-based materials.

Modular Integration of Upconverting Nanocrystal-Dendrimer Composites for Folate Receptor-Specific NIR Imaging and Light-Triggered Drug Release

Upconversion nanocrystals (UCNs) display near-infrared (NIR)-responsive photoluminescent properties for NIR imaging and drug delivery. The development of effective strategies for UCN integration with other complementary nanostructures for targeting and drug conjugation is highly desirable. This study reports on a core/shell-based theranostic system designed by UCN integration with a folate (FA)-conjugated dendrimer for tumor targeting and with photocaged doxorubicin as a cytotoxic agent. Two types of UCNs (NaYF4:Yb/Er (or Yb/Tm); diameter = ≈50 to 54 nm) are described, each displaying distinct emission properties upon NIR (980 nm) excitation. The UCNs are surface modified through covalent attachment of photocaged doxorubicin (ONB-Dox) and a multivalent FA-conjugated polyamidoamine (PAMAM) dendrimer G5(FA)6 to prepare UCN@(ONB-Dox)(G5FA). Surface plasmon resonance experiments performed with G5(FA)6 dendrimer alone show nanomolar binding avidity (KD = 5.9 × 10(-9) M) to the folate binding protein. This dendrimer binding corresponds with selective binding and uptake of UCN@(ONB-Dox)(G5FA) by FAR-positive KB carcinoma cells in vitro. Furthermore, UCN@(ONB-Dox)(G5FA) treatment of FAR(+) KB cells inhibits cell growth in a light dependent manner. These results validate the utility of modularly integrated UCN-dendrimer nanocomposites for cell type specific NIR imaging and light-controlled drug release, thus serving as a new theranostic system.

Photoswitchable semiconductor nanocrystals with self-regulating photochromic Förster resonance energy transfer acceptors

Photoswitchable molecules and nanoparticles constitute superior biosensors for a wide range of industrial, research and biomedical applications. Rendered reversible by spontaneous or deterministic means, such probes facilitate many of the techniques in fluorescence microscopy that surpass the optical resolution dictated by diffraction. Here we have devised a family of photoswitchable quantum dots (psQDs) in which the semiconductor core functions as a fluorescence donor in Förster resonance energy transfer (FRET), and multiple photochromic diheteroarylethene groups function as acceptors upon activation by ultraviolet light. The QDs were coated with a polymer bearing photochromic groups attached via linkers of different length. Despite the resulting nominal differences in donor-acceptor separation and anticipated FRET efficiencies, the maximum quenching of all psQD preparations was 38±2%. This result was attributable to the large ultraviolet absorption cross-section of the QDs, leading to preferential cycloreversion of photochromic groups situated closer to the nanoparticle surface and/or with a more favourable orientation.

Strategies for interfacing inorganic nanocrystals with biological systems based on polymer-coating

A representative set of nanocrystals made of semiconductors, Au and iron oxide, surface-capped with polymer ligands presenting various metal-coordinating groups.

Third-order nonlinear optical response and optical limiting of colloidal carbon dots

In this work, the nonlinear optical response of some organophilic and hydrophilic carbon dots derived from gallate precursors is studied under 4 ns and 35 ps, visible (532 nm) and infrared (1064 nm) laser excitation conditions by the Z-scan technique. The prepared carbon dots were found to exhibit considerable nonlinear optical response in the visible, the organophilic ones exhibiting stronger response in the infrared and, in general, significantly larger response than their hydrophilic counterparts. In all cases, the corresponding nonlinear optical parameters have been determined. In particular, it was found that both carbon dots exhibited important negative nonlinear refractivity, under all excitation conditions tried, corresponding to self-defocusing, while negligible nonlinear absorption was found in the ps regime. Oppositely, both types of carbon dots were found to exhibit sizeable nonlinear absorption under ns excitation, indicating their potential for optical limiting applications. The present results are discussed and compared with other results concerning similar carbon based nanostructures reported in the literature.

Supramolecular assemblies of semiconductor quantum dots and a bis(bipyridinium) derivative: luminescence quenching and aggregation phenomena

CdSe and CdSe–ZnS nanocrystal aggregates are formed in solution in the presence of a bis(bipyridinium) species, and disrupted upon addition of a macrocyclic host.

Highly stable intrinsically radiolabeled indium-111 quantum dots with multidentate zwitterionic surface coating: dual modality tool for biological imaging

A highly stable bimodal indium(111) radiolabeled indium QDs were synthesized for in vivo SPECT/fluorescence imaging.

Luminescent 'On-Off' CdSe/ZnS quantum dot chemodosimeter for hydroxide based on photoinduced electron transfer from a carboxylate moiety

A CdSe-ZnS quantum dot (QD) has been surface functionalised by a place exchange reaction with p-mercaptomethyl benzoate synthesized by a three-step procedure. The resulting lumophore-spacer-receptor QD-conjugate was characterized by IR, UV-visible and fluorescence spectroscopy. The emission profile of the QD reveals a narrow emission peak centred at 542 nm. Addition of hydroxide to the solution containing the QD-conjugate results in quenching of the original fluorescence, which is attributed to a photoinduced electron transfer reaction from the electron-rich benzoate moiety to the QD valence band. This is the first reported example of fluorescent quenching of a CdSe-ZnS QD luminescence by an aryl carboxylate moiety.

Photoactivatable Fluorophores

Photoactivatable fluorophores switch from a nonemissive to an emissive state upon illumination at an activating wavelength and then emit after irradiation at an exciting wavelength. The interplay of such activation and excitation events can be exploited to switch fluorescence on in a defined region of space at a given interval of time. In turn, the spatiotemporal control of fluorescence translates into the opportunity to implement imaging and spectroscopic schemes that are not possible with conventional fluorophores. Specifically, photoactivatable fluorophores permit the monitoring of dynamic processes in real time as well as the reconstruction of images with subdiffraction resolution. These promising applications can have a significant impact on the characterization of the structures and functions of biomolecular systems. As a result, strategies to implement mechanisms for fluorescence photoactivation with synthetic fluorophores are particularly valuable. In fact, a number of versatile operating principles have already been identified to activate the fluorescence of numerous members of the main families of synthetic dyes. These methods are based on either the irreversible cleavage of covalent bonds or the reversible opening and closing of rings. This paper overviews the fundamental mechanisms that govern the behavior of these photoresponsive systems, illustrates structural designs for fluorescence photoactivation, and provides representative examples of photoactivatable fluorophores in actions.

Quantum dot enabled molecular sensing and diagnostics

Since its emergence, semiconductor nanoparticles known as quantum dots (QDs) have drawn considerable attention and have quickly extended their applicability to numerous fields within the life sciences. This is largely due to their unique optical properties such as high brightness and narrow emission band as well as other advantages over traditional organic fluorophores. New molecular sensing strategies based on QDs have been developed in pursuit of high sensitivity, high throughput, and multiplexing capabilities. For traditional biological applications, QDs have already begun to replace traditional organic fluorophores to serve as simple fluorescent reporters in immunoassays, microarrays, fluorescent imaging applications, and other assay platforms. In addition, smarter, more advanced QD probes such as quantum dot fluorescence resonance energy transfer (QD-FRET) sensors, quenching sensors, and barcoding systems are paving the way for highly-sensitive genetic and epigenetic detection of diseases, multiplexed identification of infectious pathogens, and tracking of intracellular drug and gene delivery. When combined with microfluidics and confocal fluorescence spectroscopy, the detection limit is further enhanced to single molecule level. Recently, investigations have revealed that QDs participate in series of new phenomena and exhibit interesting non-photoluminescent properties. Some of these new findings are now being incorporated into novel assays for gene copy number variation (CNV) studies and DNA methylation analysis with improved quantification resolution. Herein, we provide a comprehensive review on the latest developments of QD based molecular diagnostic platforms in which QD plays a versatile and essential role.